Gemology – the science of gems

Among all gems, diamond holds a special place, as it is the only gem composed of the single chemical element, carbon. Furthermore, it is the only gem where colourlessness is esteemed more than its slight colouration. Its name derives from the Latin word adamas, meaning unconquerable. Specifically, its hardness is unassailable, as it is by far the hardest of all known minerals. Diamonds are formed very deep below the Earth’s surface and are almost without doubt also situated about 100 km below the spot where you are now reading the text on this website. They reach the surface in the shape of crystals in kimberlite magma as well as in other rocks, which are characteristically formed deep below the Earth’s surface. In recent times, the most popular diamonds in metamorphic rocks are eclogites. These rocks are also found in Pohorje Mts, although diamonds of jewellery quality cannot be expected there for the present. Historically significant diamond deposits were established in India, around 1,725 they were discovered in Brazil, and in 1870 in the Republic of South Africa. Currently, the most important diamond deposits are located in Botswana, Angola, Russia and Canada, whereas the prominent Argyle pink diamond site in Australia is now of lesser significance. From some countries, particularly African, blood or conflict diamonds are also appearing on the market. These stones travel along illegal routes run by certain organized groups, which are providing money in this way for their work that is, however, mostly associated with violence and terrorism. A few years ago, gemmologists decided to take part in the fight against this illegal trade by creating standards that enable them to determine the origin and sites of a number of diamonds.

Beryl is a mineral with the highest number of colour varieties in the world of gems. Green beryl, which is known for its three varieties, is particularly appreciated: emerald is green due to chromium admixture, green beryl is green due to iron admixture, and vanadium beryl is green due to vanadium admixture. Also known are pink morganite, yellow golden beryl, green-yellow heliodor, colourless goschenite, red beryl and blue aquamarine. Star emeralds are emeralds with asterism. A star emerald usually has a six-pointed star owing to inclusions of the mineral rutile. The most attractive star emeralds have been found in Brazil, but are so rare that even enthusiasts and connoisseurs get sight of them only exceptionally. Slightly commoner are emeralds with the image of a cat’s eye: on the surface of the cabochon, a more or less thin but distinct bright line is visible on the surface of the cabochon, travelling across the surface of the cabochon if the latter is rotated around a source of light. The reason for this phenomenon are liquid inclusions and inclusions in the form of small but mutually parallel channels, less often inclusions of larger, mutually parallel needle crystals.The intermediate space is filled predominantly by dark, almost black, fine-grained albite with some calcite. As albite and calcite are much softer than emerald, the trapezoid emerald often resembles a cogwheel. The fictitious star, formed by the albite as an inclusion between the emerald crystals, does not move across the surface of the gem as in a star emerald or cat’s eye emerald. Quite exceptional, superb combinations are trapezoid emeralds, which are also characterized by the optical phenomenon of a cat’s eye – i.e. on all six crystals overgrowing the central crystal. The most celebrated trapezoid emeralds originate from Colombia. They are incorporated into high-quality, unique jewellery, which of course has its appropriate price. The richest commercial emerald sites are the Muzo and Chivor mines in Colombia. The remaining more important deposits of emeralds are in Brazil, Zambia, Tanzania, India, Pakistan and Afghanistan, while in Europe the most important site is located in Habachtal in neighbouring Austria. The major sites of aquamarine are in Brazil, Pakistan and Namibia. The Wah Wah Mts in Utah (USA) are world-famous for their bixbyite in rhyolite, while Pakistan, Myanmar and Sri Lanka are known for other colour varieties. Bluish-coloured beryl can also be found in several places in our Pohorje Mts.

Topaz is a mineral and a gem, but somehow ignored in our country. It is interesting, however, that in spite of its different colouration it has no different jewellery names, as we have noted in the case of corundum or beryl. Consequently, topaz is a term for a mineral and a gem, although we should not forget that this term historically denoted different stones: in antiquity, for example, yellow precious stones were called topaz, while later on the term topaz referred to the yellow quartz citrine. Tapas means “heat” or “fire” in Sanskrit, while the ancient Roman historian Pliny was convinced that the gem was given its name after the legendary island in the Red Sea where supposedly  found for the first time. Topaz is colourless as well as coloured, the commonest are yellow and orange, the rarest naturally pink. Most attractive are blue topazes, although the great majority of them are processed, since naturally blue ones are very rare. Today, dealers use special terms for the colour-treated blue topaz, such as Swiss blue and London blue. Bright green topazes are treated, too, as they are never so distinctly coloured in nature. Topaz is in most cases found in pegmatites, with crystals reaching a meter or more in size. It is also found in high-temperature quartz veins, less important are deposits in igneous rocks granite or rhyolite. The most important  locality of orange topaz is in Brazil, where it is also known as imperial topaz. A  well-known locality of natural pink and blue topazes and  particular large colourless topazes is in Russia. In Myanmar and Sri Lanka, they also occur in secondary deposits in gravel bars. Their localities are also Pakistan, as well as in China where discovered only recently.

Olivine is not the term used for a mineral, but for a solid solution of two minerals: forsterite and fayalite. Hence, olivine has a variable chemical composition, meaning that its physical properties are also varying: forsterite, which contains only magnesium, is colourless. The more iron is admixed in it, i.e. the more fayalite is present in the solid solution, the greener the fayalite. It can also be dark green and almost black. In jewellery, olivine is generally transparent and green to yellow-green. Apart from the classic transparent olivine, olivine with the cat’s eyes image and stellar olivine with a four- and six-pointed star is known. Most attractive are olivines in meteorites (pallasites), where they are in strong contrast with the grey iron-nickel metal. Minerals in the forsterite-fayalite series are formed during primary crystallization from magmas, which are relatively rich in iron and magnesium and relatively poor in quartz. Dunite and peridotite rocks are formed from them. Olivine is frequently found in kimberlite, too, the rock in which diamonds are the commonest. The most important locality of olivine was once on Zabargad Island in the Red Sea. Prisoners once slogged there with no fresh water in a very desert environment, and none of them came alive from the island. The quality of the Zabargad olivines far exceeds the quality of emeralds from the pharaonic period. Hence it is not surprising that even Cleopatra esteemed olivines more than emeralds. The term olivine reflects the characteristic green colour of this gem; the name peridot is used in the US, with the name chrysolite allegedly more appropriate for the greenish-yellow varieties. The most important localities of jewellery olivine are in Pakistan, while in Myanmar they are rare. In Slovenia, olivine bombs can be found near Grad in the Goričko region.

We Slovenians can be proud of having our own representative in the world of gems. This is dravite, a brown tourmaline named after the Drava River. Black tourmalines, called schorlites, can also be found in our country. In jewellery, the most important among tourmalines are the multi-coloured varieties of elbaite. Once they were called rubellite, verdelite and indigolite on the basis of their colours. Tourmalines constitute a group of fourteen acknowledged minerals today. Their chemical composition is not simple. They are silicates, i.e. compounds of silicon and oxygen, with aluminium, iron, magnesium, potassium, lithium, sodium and some other elements joining their crystal structure. These elements, however, can be replaced in their structure. Their common name, tourmaline, originates from the Sri Lankan word turi mali, delineating a multicoloured stone. Tourmaline crystals are frequently multi-coloured and consequently highly esteemed. The colour of tourmalines, its depth and liveliness know no bounds. Minerals of the tourmaline group are amongst the most colourful minerals. Completely pure elbaite can otherwise be colourless, while schorlite is in principle black and dravite brown. Still, green and blue dravites, intensely coloured elbaites and others are also known. This is the reason why the terms verdelite, rubellite and indigolite are obsolete and unfitting today. As far as coloured tourmalines are concerned, it is more difficult to find uniformly coloured crystals than bicoloured or multicoloured ones. Even in a seemingly monochromatic crystal, at least two shades of the same colour can often be seen. It is very rare that the conditions in the crystallization of tourmalines in nature are such that their chemical composition, which directly affects the colour, does not change during their growth. The colour of tourmalines changes both along the c-axis and zonally, specifically from the centre of the crystal outward. Tourmalines are also strongly dichroic: if they are looked at from different directions, their colour changes, or its shade at least. The causes of colouration are various, most often they can be ascribed to admixtures of chemical elements. Iron has an effect on green, blue, yellow, brown as well as pink and reddish colours for the fact that with its different valence states it can occupy different parts in the crystal structure of tourmaline: divalent iron gives blue and trivalent iron yellow and brown colours. Apart from iron, at least manganese, chromium, vanadium and titanium exert influence on the colour of tourmalines.

Manganese has an effect on pink and red, chromium and vanadium on green, and titanium on dark blue to black colours. Tourmaline seldom contains copper admixtures, which make the crystal intensely blue, and in combination with manganese, iron and titanium, purple-blue-pink tourmalines are formed. Tourmalines crystallize in very diverse rocks, both igneous and metamorphic, largely slates and marbles. The most attractive and large crystals occur in pegmatites, in hydrothermal veins and hydrothermally altered rocks. As a relatively resistant mineral, it is also found in clastic sedimentary rocks, such as conglomerate and sandstone. Granite pegmatites are found all over the globe, except that those with minerals of jewellery quality are much rarer: in Brazil, California, Namibia, Nigeria, Afghanistan, Madagascar and the Urals. In Slovenia, granite pegmatites occur in the vicinity of Ravne in Carinthia, where we can find black tourmalines, mainly schorlite, in combination with dravite, accompanied with quartz and claystone.

In Slovenia, tourmalines also occur in metamorphic rocks, for example in metamorphic rocks mica shists at Dobrova near Dravograd, in which brown tourmaline dravites were found for the first time and first described in 1883 by Gustav Tschermak. Hence, the aforementioned locality is considered the locus typicus of this mineral.

Garnets constitute a v very important group of minerals with several jewellery types, with their crystals proportionately developed in all directions. They are frequently small and since they are resistant to weathering they are found in the sands of rivers and streams. They look like uniform grains, and this was the inspiration for their common name. The name garnet derives from granatus, the Latin word for grain. In the garnet group, six basic minerals are significant in jewellery: pyrome, almandine, spessartine, andradite, grossular and uvarovit. The common chemical formula of garnets is X3Y2 (SiO4)3. Iron, magnesium, calcium, and manganese are exchanged at the X position, while aluminium, iron, and chromium are exchanged at the Y position. Individual minerals of the garnet group can form a solid solution with each other. We distinguish pyralspites, which have aluminium at the Y position (proper, almandine, spessartine), and granites, which have calcium at the X position (uvarovitic, grossular, andradite). In nature, solid solutions are frequently formed by pyralspites and particularly by intercalation. For example, a solid solution of proper and almandine with the chemical formula (Mg,Fe)3Al2(SiO4)3, known as rhodolite, is quite highly esteemed. Garnets have a vitreous to semi-diamond luster and a relatively high refractive index that changes depending on the chemical composition of the garnet, similar as in density and other optical properties. Almandin 

is an iron-aluminium garnet that is always accompanied by at least some admixture of a pyrope component. It is characteristically dark red-orange to brown-red, red or even purple. It is a relatively common mineral; the most beautiful specimens come from India, Zambia, Brazil, Madagascar and Sri Lanka. Almandines with asterism are also known. They are relatively common in archaeological artefacts found in Slovenia. Rhodolite is a solid solution of almandine and pyrope. It excels due to its characteristic violet-yellow colour. Rhodolites of the highest quality come from Tanzania, Sri Lanka and Madagascar. Pyrope is a magnesium aluminium garnet and if it was utterly pure it would be colourless. But since it always contains an admixture of iron or chromium, it is red. If it forms a solid solution with spessartine, we get a colour-changing garnet: in daylight it is green, in artificial light red. It is also known as Malaya garnet. But if pyrope forms a solid solution with almandine, we get the already mentioned version of garnet rhodolite. Pyrope is purplish-red or red-purple, which is a highly esteemed colour. Once an important locality of pyrope existed in the Czech Republic, which was the reason why its trade name was “Czech Garnet”. Attractive pyropes occur in kimberlite rocks in South Africa and in many places around the globe. Spessartine is a manganese aluminium garnet and relatively rare of jewellery quality. The most popular are transparent orange to orange-red specimens. Some of them have a diamond shine. The most prominent localities are in Brazil, Myanmar, Sri Lanka and Madagascar. Grosular is a calcium aluminium garnet of generally yellow-orange, red-orange or orange-brown colours. The best known is the jewellery variety of hessonite, which is yellow-orange to reddish-orange. The most noble grossular is green tsavorite, named after Tsavo Park in Kenya, where discovered for the first time. The most important localities of grosular are in Sri Lanka and Canada. Andradite is a calcium iron garnet, occurring in yellow, green, brown or black colours. The most famous is the green demantoid. It is one of the rarest gems, but has become very popular recently. The most beautiful specimens come from the Urals. Topazolite is a translucent to transparent andradite with cat’s eyes characteristic, while the black non-transparent andradite is called melanite. Uvarovite is a calcium chrome garnet occurring in clusters of small crystals with, however, a very high luster. Its intense green colour made it highly popular. The most beautiful are from the Urals and Finland. In hydrogrosular, part of the silicon oxide is replaced by the OH group. Translucent to non-transparent specimens are green, bluish, pink, white and grey. Rainbow garnet occurs in all colours, even blue, which is something extremely rare as far as garnets are concerned. The colour is a consequence of light refraction on phantom garnet crystals. In Slovenia, garnets can be found in the Pohorje Mts and Carinthia. In most cases, they are a solid solution of almandine and pyrope. In the western Pohorje Mts, several centimetres long andradite can also be found.

Chrysoberyl is generally a poorly known mineral. Although its twin crystals are esteemed by enthusiasts and collectors, not many of them can be found in collections. Specifically, they are quite expensive. Chrysoberyl’s name originates from the Greek word chrysos (gold), which indicates its generally yellow colour. It crystallizes mainly in pegmatites or in metamorphic gneiss rocks, mica slates and dolomite marbles. The commonest chrysoberyl in jewellery is a transparent yellow variety with a glassy to semi-diamond luster. It can also be colourless or yellow-green, brown, blue-green, even red or purple. The most famous is the variety, the colour of which is different in daylight than in artificial light. It is called alexandrite, in honour of the Russian Tsar Alexander II. It is yellow-green, brown-green or blue-green in daylight, and orange-red, brown-red or purplish-red in artificial light. Apart from the highly esteemed and rare alexandrite, a famous variety of chrysoberyl is also cat’s eye (the term for an optical phenomenon in general is in our case the very name of the variety) or chymophane. The name highlights optical characteristics, nicely discernible on cabochons: a bright band of light evenly reflecting across the dark yellow to brown stone. The main localities of chrysoberyl and alexandrite are in Brazil, Russia and Sri Lanka. While Indian cat’s eye varieties are well-known, those from Sri Lanka are of higher quality.

Spinel is yet another gem, behind which solid solutions are hidden. This is why we are talking about a whole spinel group of minerals, similar as about garnet. In terms of chemical composition, the spinel group of minerals is very diverse. There are iron-aluminium, zinc-aluminium, manganese-aluminium, magnesium-iron, zinc-iron, magnesium-chromium and iron-chromium spinels. Jewellers have special liking for magnesium aluminium spinel or spinel with a very similar chemical composition. Jewellery quality spinels are found predominantly in marbles or at secondary localities in stream and river sands. Hence, they are often confused with ruby, for the rounded ruby and spinel grains are difficult to distinguish with the naked eye. Red spinels are particularly esteemed, followed by blue, pink, orange, purple, yellow, brown, black and colourless spinels. Colour is the consequence of chemical composition, which also exerts influence on other optical properties. Gahnospinel, for example, which is a greenish-blue solid solution between spinel MgAl2O4 and gahnite ZnAl2O4, has a considerably higher refractive index and an anomalous double refraction. Red, pink, orange, green and nearly colourless spinels frequently show strong fluorescence. Red and orange colours are brought about by an admixture of chromium, while cobalt makes spinels blue. They change colours very rarely; the characteristic of asterism is somewhat more common.

There are specimens with a six-pointed star and a twelve-pointed star, which are the consequence of the oriented intergrowth of spinel with rutile. The main spinel localities are in Brazil, Myanmar, Afghanistan and Madagascar. 

As a mineral and a gem, spodumene is highly esteemed in some places, while in Slovenia it is relatively rare in jewellery. It is a very gentle stone, though hard enough to be more than just useful. Its name originates from the Greek word spodumenos, indicating its ashen grey-white appearance when exposed to higher temperatures. The first specimens of jewellery quality were discovered in 1870 in Brazil as transparent yellow crystals with a glassy lustre. Nine years later, green spodumene crystals were found in North Carolina (USA) near the town of Stony Point and were eventually named hiddenite after the local mine’s supervisor W. E. Hidden. At the same time, the town itself was renamed Hiddenite, which remains the world’s most important locality of this stone. Gentle pink to light purple specimens were found in California and named kunzite after the gemmologist G. F. Kunz. While some kunzites fluoresce orange, hiddenites fluoresce very rarely, but if they do, they fluoresce feebly orange-yellow. Kunzites obtain their pink colour from manganese admixture, while their intense green or greenish-yellow colour is due to iron, very rarely chromium. The rarest kunzites are blue, something special are spodumens with the cat’s eye image. All coloured varieties have a distinct pleochroism. Some of the more intensely coloured kunzites gradually fade or even completely lose their colour if exposed to daylight. As spodumene is known for its good cleavage, it is not found in secondary localities, as it crumbles earlier. Owing to its good cleavage, it is also hard to grind. It occurs in granite pegmatites, together with other lithium minerals. Spodumene is an important industrial mineral, with its deposits also located in Brazil, Madagascar, Afghanistan and Pakistan. It is interesting to note that crystals weighing even up to 90 tons have been found! Jewellery quality spodumens, however, are considerably lighter and smaller, yet highly esteemed for their gentle colour.

Zoisite holds a very special place in Slovenia and in our cultural heritage. Samples of this mineral were found by Simon Prešern, a mineral dealer, on Svinška planina in the region of Carinthia. He passed them on to Baron Sigmund (Žiga) Zois, and as the latter supposed that he had a new mineral in his hands, he sent the samples on to various European mineralogists. They confirmed his supposition and named it zoisite in his honour. Today, green zoisite in the rock aniolite, pink zoisite thulite with an admixture of manganese,and tanzanite, the most precious zoisite from Tanzania, are also known. The first examples of this prominent variety were discovered by geologists when mapping the territory following the great fire in Tanzania. They found blue-violet crystals of zoisite, but as soon as they opened the mine, they realized that the crystals were in fact colourless or grey and white. The crystals acquired their bluish-purple colour owing to the increased temperature. Practically all blue-violet tanzanites are even today additionally heat-treated. The procedure is permitted, for it imitates processes in nature. Apart from it, their colour is permanent.

Jade is the trade name for green translucent to non-transparent minerals. More than 3,000 years ago, a green stone called yu was used in China during various ceremonial rituals and for stone tools and weapons making. Today it is known as nephrite. As late as the eighteenth century, another green stone called jadeite was discovered in China, where brought by traders from Myanmar. For both minerals, their common trade term jade is used, although many other green minerals can be hidden under this name today. Jadeite belongs to the pyroxene group of minerals. It is generally massive, granular or fibrous. It is found in serpentinites, which are formed during the metamorphosis of rocks rich with sodium. The most characteristic is green jadeite which, however, can be very colourful, colourless, white or black. The most esteemed jade is the noble green variety with its trade name imperial jade. It has a lively apple green colour, is non-transparent to slightly translucent, and very rare. Also popular are purple jadeite and jadeite, which is a solid solution with diopside CaMgSi2O6 and called chloromelanite. It is semi-translucent to non-transparent, green to black, depending on which pyroxenes predominate in it. It is also known as maw sit sit. Jadeite’s green, yellow or brown colours are acquired from an admixture of iron;  the most attractive intensely green jadeites obtain their colour from an admixture of chromium. The most famous jadeite localities are in Myanmar, where the only maw sit sit deposits are also situated. Other localities are of lesser importance, but let us mention at least the localities in Russia, Guatemala, New Zealand and Japan. Jadeite is the highest valued of all the stones classified as jades. The most pertinent is the colour, then the originality of the design and engraving of unique decorative items. Owing to its popularity and rarity, it is replaced with other stones or is additionally coloured. Jadeite was named after the Spanish term for this stone, piedras de ijada. When the term was translated into French as pierre de l’èjade, a misprint occurred, altering it into pierre de le jade, which the English hastily translated into jade, while in our country we named it žad (zhad). Nephrite belongs to the amphibole group of minerals and is one of the solid solution minerals of tremolite and actinolite. It crystallizes in the monoclinic system and is, like jadeite, generally massive or occurs in fibrous crystals. Tremolite is mostly  formed during metamorphosis in dolomite marbles, whereas actinolite is characteristic of schists. Nephrite is green owing to iron impurities, although it is also brownish, yellowish, greenish-yellow and black. The admixture of magnesium gives it creamy colour. Cat’s eyes, which are the consequence of parallel inclusions of fibrous iron actinolite, are rare. The best known jade deposits are in China, Turkestan, Lake Baikal, New Zealand and some other countries. Nephrite is much more common than jadeite. Its name originates from the Greek word nephros, meaning kidney, thus designating the shape of the rounded nephrite pebbles, which are fairly common in Asia. Other, especially green minerals, have also been classified among jades by some people. One of these is omphacite, one of the indispensable minerals of the metamorphic rock eclogite. In rare cases, the stone is preserved to such extent that it can be used for decorative purposes. Specifically, green omphacite is in a beautiful contrast with blue kyanite, pink corundum and red garnets. In addition to the listed minerals, colourless, grey or white zoisites can also be found in some places.

Quartz is one of the commonest minerals on the surface of our planet and a very abundant decorative stone. It holds a special place among minerals. Quartz was once believed to be so deeply frozen water that it could no longer melt. Its crystals became a symbol for the rock crystal, and in some parts of Slovenia crystals are called mini churches owing to their recognizable shape. Quartz can be roughly separated into fine-crystalline and coarse-crystalline quartz. It is characteristic of microcrystalline quartz that never occurs in crystals, the flat facets of which can be admired with the naked eye. These include agates, jaspers, chalcedony and a series of other varieties. Coarse-grained quartz includes quartz, which develops attractively shaped crystals that can be admired with the naked eye. Thick-crystal varieties of quartz are very useful for decorative purposes. The colourless variety is called stone lightning. If quartz contains several tiny liquid inclusions, it looks milky, which means that it is no longer transparent but translucent to semi-translucent and white. It is called milky quartz. Owing to irradiation with natural radioactive rays, the structure of quartz is moderately deformed. The result is brownish quartz, called smoky quartz. If it is dark brown or almost black, we are dealing with morion quartz. The admixture of divalent iron in the structure exerts influence on the yellow colour of citrine, while trivalent iron has an effect on the purple colour of amethyst. By heating amethysts, we can thus get citrines or green quartz, erroneously called prasiolite. If yellow and violet colours alternate in a single crystal, we are dealing with ametrine. The chemical element titanium gives rise to pink colour, and this variety is called rose quartz. Quartz with asterism is mostly the consequence of oriented intergrowth of quartz with rutile. In rose quartz, this optical phenomenon is clearly visible in light transmission. Quartzes with the optical image of cat’s eyes are also common, the most favoured being those with rutile or tourmaline. A fine-grained variety of quartz with microscopically small fibrous crystals is called chalcedony. The main characteristic of this variety is that it never develops crystals large enough to be seen with the naked eye. It is formed in sedimentary, metamorphic and igneous rocks. It grows in different colours and shapes. Individual varieties contain up to 2.5% water and/or opal. Chrysoprase is the noblest variety of chalcedony. It is characteristically apple green owing to the admixture of vanadium. Carnelian is translucent to non-transparent and orange to red. The uniform orange parts of carnelian were frequently used in gem making during the Roman period. The translucent to non-transparent red-brown variety is sard. The distinct nature of sardonyx is the alternation of at least two red-brown-white and white-black layers of fine-grained quartz. Onyx is generally a non-transparent chalcedony with alternating black, grey, brown and white streaks. It is much used for engraving, and often only its black and white or only black parts are polished. Heliotrope is a non-transparent green variety of chalcedony with minute inclusions of red jasper. Chalcedony also grows into chalcedony roses, which are sometimes sprinkled with tiny quartz crystals, which makes them fairly attractive. In a narrower sense, chalcedony is also known as a translucent bluish variety of fine-grained quartz with alternating light bluish and white streaks. Agates are among the more important fine-grained varieties of quartz. They are generally formed near volcanoes and are almond-shaped and millimetre or even metre in size. They are particularly attractive in cross-section, when separate layers are visible; if crystallization is stopped, quartz crystals crystallize in the central part of the agate geode. In agate patterns, nature found vent, for these are occasionally truly unique. But let us not forget that agates are also artificially dyed, sometimes in unnatural, gaudy colours. Sometimes it seems as if plants were caught in the stone. Such finely grained varieties of quartz are mostly known as moss agate and dendritic agate. The first is generally colourless or white chalcedony, in which green dendrites of chlorite are caught. A particular feature of the second are inclusions of black dendrites in light to non-transparent chalcedony. Fire agate from Mexico has distinct interference colours, which are the consequence of light refraction on inclusions of iron hydroxides excreted between individual layers of chalcedony. Jasper is a fine-grained variety of quartz with microscopically small quartz crystals. The same as in chalcedony, the jasper crystals are never so large to be admired with the naked eye. Jasper contains up to 20% of other minerals that render it the characteristic image. It is usually red, but there are also green, brown, yellow, blue – in fact, it can be found in all colours. More nice-looking specimens have inclusions of pyrite or hematite. Plasma is jasper that acquired its green colour due to inclusions of green minerals (such as celadonite). Prasem, too, is a green variety of jasper, except that its colour is obtained from inclusions of actinolite. Pseudomorphosis of quartz replaced by other minerals is common. Most often, the petrified trunks and their twigs can be admired. Hawk’s eye is a bluish-grey pseudomorph of quartz replaced by crocidolite, while tiger’s eye is pseudomorph replaced by oxidized crocidolite, which makes it yellow-brown. Both can have the optical image of cat’s eye. Quartz localities are found worldwide. They are  known particularly from Brazil, the Himalayas and the Alpine countries. In Slovenia, amethyst occurs in several places in the Pohorje Mts, stone lightning at a number of localities (Hrastnik near Škofja Loka, Črni Vrh above Polhov Gradec, Sostro near Ljubljana, Cerknica, Znojile, Pohorje Mts), smoky quartz in the vicinity of Žirovski Vrh, Krgana near Škofja Loka and in Pohorje Mts, a special feature is quartz with rutile inclusions in the vicinity of Krašnja. Finely grained quartz is found all over the world. Deposits of chrysoprase are found in Poland and Australia, fire agate in Mexico and historically important carnelian deposits in Egypt. In Slovenia, jasper occurs in several places, the best quality jasper is found around Velenje, Idrija and Vrhnika. White, green and semi-transparent chalcedonies occur in several places in the Pohorje Mts. Onyx is common, but most often so badly cracked that it is not useful for jewellery purposes. Around Turjak, carnelian can be found simply in the fields. Undoubtedly, however, new Slovenian deposits localities of finely grained quartz useful for jewellery will be discovered.

Opal is quartz without a crystal structure. Or, to be more specific: opal is an amorphous form of quartz with water. The most frequently occurring is common opal with no characteristic play of colour, while precious opal always has it. The iridescent colours are brought about by light refraction on the small beads of cristobalite or tridymite, of which opal is also made up. Depending on their size and relationship with each other, all rainbow colours can be achieved. The most esteemed is red, followed by purple, blue and green. In jewellery, varieties are named in accordance with the shape and basic colour, or with the colour and colour patterns that co-create the image of opal. The basic colour of opals ranges from white to grey to almost completely black. This is why we call them white, grey or semi-black and black opals. Fire opal is a transparent yellow-orange or red opal with or without play of colour. Numerous opals are translucent to transparent and are basically colourless or grey. Such are hyalite, which is generally without opalization, crystal opal, which usually has intense, and hydrophane, which becomes translucent to transparent and with a more intense play of colour, if immersed for some time in water.

Opals, which alternate with fine-grained quartz in nature are common. Therefore, almost all finely grained quartz varieties always contain at least some opal – and vice versa. If opal predominates, specimens of this kind can be considered as opals: jasper opal, pars opal, and the like. A special case is precious opal with a special colour pattern or shape, often characteristic of a single locality: boulder or matrix opal is precious opal in a fairly limonitized rock. Polished specimens look like a thin layer of opal on a brown background; andamooka opal is mined in Andamooka, Australia; harlequin or mosaic opal has colour fields in the form of adjacent rectangles or rhombuses; patterns of flame opal resemble flames of fire; in flash opals, fields of colour appear and disappear rapidly as the gem turns; peacock opal is characterized by patterns of blue and green shades, while lechosos opal is characterized by intense play of green colours. Quite rare are star opal with the three-pointed star and imperial opal with the cat’s eye characteristic. Common opal is found in the Pohorje Mts. Precious or noble opal, which is characterized by play of colour, was mined in Slovakia (Dubnik) before its deposits were discovered in Australia. Today, the country with the greatest amount of opals is still Australia. The most popular and affordable are those from Ethiopia. All expensive opals must be meticulously stored: they should not be exposed to direct sunlight or any other source of heat. Specifically, if they lose water, they also lose play of colour and, in turn, their essence. They should never be cleaned in ultrasonic bath.

Andalusite is a mineral and a gem occurring in the shades of autumn colours. One of its main characteristic is the highly distinct and attractive pleochroism in green-red stones. Its name was acquired from the Spanish province of Andalusia. It is a polymorphic mineral with sillimanite and kyanite. It most frequently occurs in metamorphic rocks, very rarely in pegmatites and granites. In gemmology, it is a common transparent variety with a glassy brownish, brownish-red, and green lustre. The andalusite’s green colour is the consequence of manganese admixture, while its dark green varieties are called viridine. The red colour is brought about by iron admixture. Less often, it is yellow, pink, blue or colourless. The popular non-transparent to translucent andalusite variety with a cross-shaped pattern transverse to the crystallographic c axis of the crystal is called chiastolite. The andalusite’s basic colour is in this case yellow, grey, white or reddish, while the inclusions that render it the image of a cross are of graphite and thus always black. This is why this variety is somewhat softer than the transparent varieties. It was named after the Greek letter χ (chi), which indicates the graphite pattern in it. Andalusite with the cat’s eye image is very rare. The main localities of andalusite are, apart from the classic deposit in Adalusia, elsewhere in Spain, while secondary deposits are found in Sri Lanka and Brazil. Blue andalusite is known from Belgium, chiastolite deposits from Australia, France, Russia and Zimbabwe. In Slovenia, it occurs in the Pohorje Mts, but it is not of satisfactory quality to be utilized in jewellery.

Zircon is a common mineral in igneous rocks, but only few of them are of gemstone quality. It acquired its name from the Arabic words zargun, meaning gold, and gun, meaning colour. Zircon practically always contains admixtures of the radioactive elements uranium and thorium. Over millennia or millions of years, their radioactive radiation gives rise to changes in their structure. High zircon, also called alpha zircon, is a common zircon crystallizing in the tetragonal system. Low zircon, also called gamma zircon, has a collapsed structure. Therefore, it is practically amorphous or has an isotropic character. Intermediate zircon is the zircon that is partially still crystallized and has partially already turned to an amorphous form. Zircon is reddish-brown or yellow, grey, green, red and very rarely blue – in such cases, it is distinctly multicoloured. It is characterized by a glassy to diamond lustre, relatively high dispersion and a high refractive index, giving the gem a vibrant appearance. The main zircon localities are in Cambodia, Australia, Myanmar, Brazil, Sri Lanka and Thailand. In Slovenia, it is found in igneous rocks as an accompanying mineral, which regrettably cannot be seen with the naked eye.

Lapis lazuli is the only gem with a two-word name. It is also one of the few gems that is not a mineral but a rock. It consists of the minerals laurite, calcite, pyrite and some other minerals. Its name originates from the Persian word lazhward, denoting its blue colour. Lazurite is a solid solution with haüyin, sodalite and nosean, and the main mineral accountable for the lapis lazulis’ noble blue appearance. It is found in limestone and marble, not so often in granite. Crystals are relatively rare. More common is the massive or in veins-occurring lapis lazuli. It is non-transparent and light to dark blue, blue-green and also purplish-blue. In jewellery, the light blue variety is esteemed, while the dark blue specimens are less popular. Pyrite inclusions can be attractive, but if there are too many of them, they exert influence on the stone’s strength. Calcite is, next to lapis lazuli, generally white and not particularly desirable. The main localities of lapis lazuli are known from Afghanistan, Chile and Russia. For thousands of years, it has been valued for its characteristic blue colour. Until the nineteenth century, blue pigment was made from it. It can be colour treated in order to make the inclusions of white calcite not conspicuous. Its imitations are often found on the market.

Turquoise is distinguished by its light blue colour. It is a gem, which was used more than 4,000 years ago in Egypt. It was also esteemed and respected by the ancient peoples of Central America. Owing to its historical role, it holds an important place among jewels. Turquoise is the only important phosphate gemstone. Finely crystalline turquoise fills small cavities in porous rocks or forms veins. It is formed by the percolation of groundwater, which washes away rocks rich in aluminium and copper. Its colour is obtained by copper, it is light blue, rarely dark blue or blue-green. Due to the admixture of iron, it is also apple green. It has a characteristic waxy sheen and is always non-transparent. Inclusions of pyrite or limonite are common. The main deposits are known from Iran, Tibet, China, Uzbekistan, Afghanistan and several places in the USA. In nature, massive turquoise is often porous. That is the reason why it is impregnated with artificial resin and additionally colour treated. It should never be cleaned in an ultrasonic bath. Imitations and artificial turquoises are common, which makes them relatively difficult to be distinguished from natural specimens.

Malachite is one of the most attractive and popular gemstones. It is distinguished by the bands of light alternating to dark green malachite, forming attractive patterns. Malachite is often combined with azurite, and these two gems even replace each other, thus creating unique images in the world of gems. Malachite seldom occurs in radiant crystals, but is more often massive or in stalactite form. Specifically, it is formed in a very similar manner as flowstone from calcite in karst caves. The only difference is that the hanging wall rocks above the malachite stalactites are rich in copper minerals, mostly sulphides. The lustre of malachite depends on its form: crystals have semi-diamond to glassy lustre, while the massive and stalactite crystals are waxy and rarely glassy. The colour of malachite depends on the admixture of the component of clay and varies from light to dark green. During polishing, no dust should be inhaled, which means that sanding discs should be wetted more intensely. It is never to be cleaned in ultrasonic bath. Many years ago, important malachite and azurite deposits were those in Romania. Today, some important localities are known from Zambia, Namibia, Congo and Russia. In Slovenia, small crystals of malachite can occur in several abandoned mines, while radiant clusters of crystals of up to 5 mm in size can be found near Ortnek in the Dolenjska region, but none of them are of jewellery quality.

Diopside is a fairly common mineral around the globe. It is rare in jewellery but occurs in attractive varieties. Although its crystals are common, they are seldom perfectly formed. It is rarely colourless, but mostly green, yellowish-green, dark green, brown or even black. It is found mainly in metamorphic rocks rich in calcium. Diopside is a solid solution with the end member hedenbergite; then it can be completely dark, mostly black. It is usually non-transparent, though utterly transparent specimens are known as well. The bright green diopside, which is generally transparent, has an admixture of chromium and this is why it is called chromium diopside. Massive, translucent to non-transparent blue varieties are called violane. Dark brown to black diopsides with a four-pointed star effect and dark green specimens with the cat’s eye image are known, too. The most important deposits are at secondary localities in Myanmar, Canada, Kenya, Sri Lanka and Madagascar, while in Slovenia they occur in several places in the Pohorje Mts, but are of no jewellery quality.

Hematite, or red ocher, was used by American Indians and other ethnic groups as a colour base for painting bodies and chiefly faces. Later on, hematite turned into one of the most important iron extraction ores, while today it is also widely utilized for decoration. It acquired its name from the Greek word hema (blood), indicating the characteristic red line it leaves on a porcelain tile. The most famous hematite crystals occur in the form of the so-called iron flower. It is often massive and finely grained. Non-transparent grey hematite with a metallic sheen is the commonest, but it can also be found in black, maroon and pure red colours. Irrespective of its colour, it always leaves a red line on porcelain tiles. All those who have attempted to cut, grind or polish hematite know what the red mark, left during processing, means. Hematite is most often used as a decorative stone in cabochons, beads (for necklaces), et cetera. Specimens in which grey and red layers of hematite alternate are interesting indeed, but are especially esteemed when combined with yellow rutile and colourless quartz. Hematites of this kind are found in Brazil, while deposits of grey-red hematite are common all over the globe in sedimentary, igneous and metamorphic rocks, even in lava. Hematite is not rare in Slovenia either, for it can be found at Kopitov grič above Borovnica, north of Dravograd and in the already abandoned Sitarjevec mine near Litija. It is a popular stone, which is the reason why it is produced artificially as well; when buying it, carry a magnet with you: natural hematite is not magnetic, whereas the majority of artificial products are very strongly magnetic.

Chrysocolla is one of the rare minerals and even rarer gems carrying a female name. It is generally massive or very finely crystalline. It occurs generally in the oxidation zones of copper mines. It is blue, green or turquoise and multi-shaded. It is always accompanied by copper, iron and manganese oxides, which render the characteristic appearance to the stone that is seemingly non-transparent and with a waxy sheen. If chrysocolla is calcined, it has a glassy lustre and high hardness owing to quartz, but if accompanied by jasper, it emits a greenish-blue colour with red-brown spots. It occurs together with malachite, less often turquoise (this variety is called eilat stone). The chrysocolla’s main deposits are in Arizona, New Mexico, Russia, Mexico, Israel, Chile and Congo.

Rhodochrosite is a popular decorative stone owing to its gentle rose-red colour and diverse forms. Its name was acquired from the Greek words rhodon, meaning pink, and chrosis, meaning staining. Part of manganese in rhodochrosite can be replaced by iron, calcium, zinc or magnesium, due to which it acquires a different colour. Owing to this variable chemical composition, it has different optical and other physical properties. It has a glassy or pearly lustre and a highly variable index of refraction. The most attractive crystals are formed from hydrothermal solutions or as a secondary mineral in manganese ore deposits. Rare are the utterly transparent and intensely red-coloured specimens, which can also be faceted. If finely grained, it is suitable for engraving. Major localities of rhodochrosite are in Argentina, Colorado and several places in Africa. Rhodochrosite is sought after in particular by gem collectors, considering that it is too soft for wider use in jewellery; it is suitable only for brooches and parts of jewels not exposed to mechanical impacts. It should never be cleaned in an ultrasonic bath.

Rhodonite is in places a very popular decorative stone. Apart from its pink colour, it is main characteristic is the relatively high hardness. Its name originates from the Greek word rhodon, denoting the pink colour of this mineral. Rhodonite belongs to the group of pyroxenes. It is generally massive or finely grained. It is found in larger blocks here and there. It has a glassy lustre and is usually non-transparent. Its colour varies from light pink, red, maroon to purplish-pink. Most attractive specimens are strewn with black veins of manganese minerals. The intense pink transparent specimens are rarer and can be faceted, too. The rhodonite’s best known localities are in California, Canada, England, Mexico, South Africa, Russia, Sweden, Japan and Australia.

Sodalite is one of the minerals from the sodalite group, which is composed of feldspar substitutes. In view of their chemical composition, they are very similar to cleystones, except that less silica occurs in them. Besides lazurite, which is the principle mineral of lapis lazuli, sodalite is the most important mineral in this group. Its name indicates the presence of sodium. Sodalite crystals are extremely rare and generally non-transparent, sometimes translucent. The lustre is glassy to oily, the colour dark blue to purplish blue. The white variety is hackmantite, which is sometimes yellow due to its sulphur admixture. Calcite veins are common in its blue variety. Sodalite is generally a relatively rare mineral, found in nepheline syenite and other rocks with little free silica. Its deposits are located in Canada, Namibia and Brazil.

Meteorites are rocks originating from space. Most of them circle in the asteroid belt around the Sun between the planets Mars and Jupiter. Owing to their collisions with each other and especially to the large Jupiter, they digress from their usual paths and can fall on some other asteroids, planets and their moons. They are 4.6 billion years old and are silent witnesses to the formation of our solar system. There are stone, iron nickel and mixed meteorites. In jewellery, palasites are especially esteemed. These are metallic iron-nickel meteorites with large yellow to yellow-green olivine crystals.

Tektites are not meteorites, but meteorite glass. When a meteorite hits the Earth, its molten rocks with large quantities of quartz burst out, scatter widely and solidify in the air. Hence, the shape of the tektites is more or less aerodynamic and they are not found in craters but hundreds of kilometres away from them. The main localities of tektites are in the Czech Republic (moldavites), Libya (Libyan glass), Vietnam, Indonesia (indokinites), Australia (Darwin glass), Ivory Coast and in the American states of Georgia and Texas.

Obsidian is the most common natural glass, used already for making stone tools and particularly arrows and other sharp weapons. It is named after its alleged discoverer Obsius of Ethiopia. Obsidian is amorphous quartz with an admixture of calcium, sodium, potassium and other oxides. It occurs in the vicinity of the (once) active volcanoes. Specifically, it is formed through rapid cooling of lava of rhyolite composition. Most often, it is black and seemingly completely non-transparent. It can also be grey, black-grey or black and brown. Here and there it has inclusions of white cristobalite. Inclusions of pyroxenes, other minerals and air bubbles can colour it green, blue, red or purple. Specimens of this kind are called rainbow obsidians. Something special is the black obsidian with inclusions of fibrous silver or gold crystals. Seldom it is translucent and even transparent, when also suitable for faceting. Although obsidian is common, its most important localities are in Mexico, in several US places, and on Aeolian Islands in the Mediterranean. Obsidian is sensitive to rising temperatures, which should be taken into consideration when polishing and using it as a decorative stone.

The commonest minerals in the Earth’s crust are feldspars, among which jewel varieties are also present. In this large group of rock-forming minerals, potassium and sodium calcium can be roughly distinguished. They play a significant role in the jewellery industry, as they possess some interesting optical properties: labradorescence, opalescence and aventurescence. The commonest are labradorite, amazonite and moonstone. The group of potassium feldspars includes orthoclase, sanidine, microcline and anorthoclase. Orthoclase feldspars are usually colourless or white, green, brown, grey and black specimens, while the most esteemed are yellow, orange and transparent specimens that are of greatest esteem in the jewellery industry. The most popular potassium feldspar in jewellery is moonstone, a transparent to translucent high-temperature variety of orthoclase potassium feldspar overgrown with albite. This is the reason why the characteristic optical phenomenon of adularescence can be seen on the surface of the cabochons. Stellar moonstones with a four-pointed star and the cat’s eye image are also known. Sanidine crystallizes at high temperatures and it is white or yellow owing to its iron admixture. In the jewellery industry, colourless sanidines are also used. Due to its overgrowth with oligoclase, a similar iridescence of colours as in labradorite can be observed. Microcline is a typical low-temperature feldspar, while its jewellery variety is amazonite. It can be light to dark green or even blue-green. The colour is the consequence of defects in the structure when potassium is replaced by lead. Colourless or white microcline occasionally contains inclusions of needle-shaped dark green to black arfvedsonite. The colour contrast and adularescence contribute to this gem’s distinct image. Sodium-calcium feldspars or plagioclases are much more diverse in respect of their chemical composition. They are a solid solution of albite and anorthite, while in respect of their quantitative ratio – each further member has less calcium and more sodium component – the intermediate members are oligoclase, andesine, labradorite and bytownite.    

Labradorite is famed in the jewellery industry as it is basically black and coloured by interference colours in the entire rainbow spectrum. When the colours are particularly distinct, we are dealing with spectrolite. Plagioclases have variable values of refractive indices and densities in compliance with their different chemical compositions. Sunstone has a unique appearance. This is usually brown oligoclase or labradorite with tiny inclusions of hematite or geothite, which glimmer in reflected light and create aventurization. Oligoclase, andesine, bytownite and anorthite are used in the jewellery industry to a lesser extent. Completely transparent and red specimens or specimens with intense adularescence are also known. Among the barium feldspars, let us mention only hyalophane. It can be a solid solution with other barium feldspars or with orthoclase. It is generally colourless. Fairly important localities are known from Switzerland, Austria, Bosnia and Brazil.

Among other substances of organic origin, amber or fossil resin is no doubt important as well.  It is particularly attractive if an insect or some other animal or plant has been trapped in it. Till now, only a few rare examples of natural untreated amber have been found in Slovenia – for example, in the Velenje coal mine and near Vransko. More amber, however, can be found in archaeological artifacts, as the famous amber route led through our region in the distant past.