Rocks consist of different minerals, each with a definite chemical composition and atomic structure. Rocks can be divided into three main groups:

Sedimentary rocks: Weathering, transportation, deposition of sediments. Of importance in terms of gemstones is alluvial gravels for diamond, agate and garnet deposits;

In South Africa, the main “gem-containing” group is the igneous rocks. Granite is a slow- cooled igneous rock consisting of the minerals quartz, feldspar and mica. The last phase of cooling and crystallisation in a water-rich environment is known as a pegmatite. This rock is very coarse grained and concentrates rare and incompatible elements such as beryllium, boron, lithium, etc. Pegmatite bodies often host gemstones such as beryl, emerald, topaz and tourmaline. Unfortunately gemstones are not evenly distributed within the pegmatite body but occur in cavities (“pockets”) or in certain zones. This makes the exploration and evaluation of gem-bearing pegmatite deposits very difficult.

Important pegmatites that contain gemstones are located in the Northern Cape (extended belt of 450 km in length and 50 km wide in the Steinkopf-Vioolsdrif, Kakamas-Kenhardt areas); Mpumalanga (associated with Archaean Greenstone belts, Gravelotte-Mica area); Northern Province (Palakop, Pietersburg and Piet Retief areas).

Geological occurrences: Pegmatites associated with acid magmatic rocks and metamorphic schists (metamorphic rock that can split into thin flakes). Beryl has a hardness of 7,5-8 on the Mohs scale of hardness (talc=1 and diamond=10), but is brittle. The colour is translucent and white, but the presence of elements in the crystal structure gives rise to different colours and therefore to the different varieties. The different gem varieties are: emerald (green), aquamarine (blue-green), morganite (pink), heliodor (yellow), goshenite (colourless) and bixbite (red). Beryl occurs in pegmatites of the Northern Cape with colours varying from yellow-green, through greenish white, to pink for beryl in lithium-rich pegmatites.

This dark-green variety is the most valued of all the beryl varieties. It is usually included and cracked, but the appearance can be enhanced by oiling the stones and, in some cases, filling the fractures with a filler (usually glass).

Because of its brittleness it is almost always fashioned with octagonal step-cuts to reduce the risk of chipped corners. The step cut also lightens the dark body colour of the emeralds for a better appearance. Emerald occurs where pegmatites “invade” ultramafic rocks (schists). At this contact,
emerald crystallises as the result of metasomatism during regional (low grade) metamorphism.

Emeralds are also synthesised by hydrothermal and flux-grown processes, and these synthetics can be distinguished from natural emerald by unnatural inclusions. Green glass and composite stones are still used today to imitate emerald. Emeralds are mined at the Cobra Emerald Mine near Gravelotte by both excavation and underground operations. Although top-quality flawless and vivid green stones had been recovered, most
of the material is small, varies in colour and is cracked.

Gem-quality aquamarine is transparent, flawless, and has a colour resembling that of sea water (blue-green). Pale-blue and green stones also occur. Heat treatment of green stones can result in a darker blue colour. Glass and synthetic spinel are generally used to imitate aquamarine, but synthetic aquamarine has been manufactured for experimental purposes.

Aquamarine occurs in Namaqualand (near Jakkalswater) and the Murchison Range, east of Leydsdorp.

This variety of beryl has a rose-pink colour, although peach-coloured and orange stones also exist. Gemquality morganite is found in the Namaqualand pegmatites and in the vicinity of Leydsdorp, Northern Province. Heliodor: It is also known as yellow or golden beryl and is found in the Soutpansberg district, between Messina and Pietersburg in the Northern Province.

Azurite Cu2(OH)2(CO3)2 and Malachite Cu2(OH)2CO3 Azurite and malachite are commonly associated with the weathered zone of copper deposits, although these seldom occur in sufficient quality or quantity to be exploited as a gemstone in South Africa. Azurite is a dark-blue copper carbonate, semitranslucent to opaque. It is too soft and usually too dark to be faceted and is sometimes replaced by malachite.

Malachite occurs mostly as layers of alternating lightgreen and dark-green bands in massive or botryoidal bodies. It seldom occurs as crystals. The material is too soft to facet, but is cut as cabochons and beads for jewellery, or used for carvings. It has been reported from the mines at Messina and at Stavoren (Potgietersrus district), Okiep, Postmasburg and in the vicinity of Pilgrim’s Rest.

Sapphire and Ruby Geological occurrence: In pegmatites, schists, marbles, gneisses (metamorphic, coarse- grained rock with alternating layers formed by metamorphism) and alluvial gravel deposits.

Pure corundum is colourless, but usually traces of other elements colour the material. The presence of chromium oxides gives it a red colour resulting in the formation of ruby. The term sapphire is used to describe all varieties of corundum other than the red. The blue colour is due to Fe+2 and Ti+4 ions in the lattice. Gem-quality stones are always faceted. Translucent to opaque corundum is usually cut as cabochons, and poorer-quality material is used as abrasives. The appearance of rubies and sapphires is sometimes enhanced by artificial processes such as heat treatment. Many of the stones seen in the trade are actually synthetic corundum, manufactured by the Verneuil process which produces inclusions that show the synthetic nature of these stones. Rubies are also imitated by red glass, synthetic spinel, garnet and composite stones with natural and/or synthetic parts. Sapphires are usually imitated by cobalt-coloured glass and synthetic spinel. Corundum is found over a wide area including the Mpumalanga Province, with potential deposits near Badplaas (dark, translucent sapphires), Namaqualand (pink, yellow and crimson varieties in the Kenhardt district) and south of Augrabies (poor-quality sapphires and corundum). At Malalane, corundum occurs in gneiss (metamorphic rock) where the host rock is green mica (fuchsite). Although no transparent (gem quality) corundum is found at these localities, they are still important as collectors specimens.

Geological occurrences: Among the most important rock- forming minerals, in all granites and most igneous rocks and gneisses. A few varieties of feldspar are considered to be gemstones. A number of closely related minerals belong to this group.

Consists of alternate layers of the two feldspar minerals, orthoclase and albite. The interference of light by these thin layers produces the sheen, or schiller, for which moonstone is prized. The thickness of these layers determines the colour, which may be white, bluishviolet or blue. The lastmentioned colour is highly prized. Moonstone also displays chatoyancy and is cut as cabochons. Moonstone is found at the Eagle’s Nest Mine (Barberton district), Mpumalanga Province.

The colour of this variety is green to blue-green, it being translucent to opaque. Like all feldspars, amazonite has two directions of good cleavage. Fashioned stones may break along these directions if not handled with care. Amazonite occurs as wellformed crystals to massive material. Gem material is used for beads and cabochons, but not for carvings because of the prominent cleavage. Northern Province: Dark blue-green amazonite is found on the farm Honeydew 86 MR, Pietersburg district and in the vicinity of Leydsdorp. Northern Cape Province: Localities include those at Pofadder, Kenhardt district, Gordonia district, between Steinkopf and Vioolsdrif and at Augrabies Falls National Park. Unakite: Composed of pink feldspar and green epidote. Cut as cabochons. Occurrences at Upington and Neilersdrif, Northern Cape.

Garnet General formula:

L3M2(SiO4)3
L = Mg, Fe, Ca, Mn
M = Al, Fe, Cr

Geological occurrences: Garnets are found in metamorphic and igneous rocks, and as rolled pebbles in alluvial deposits. Gem-quality garnets are
transparent to translucent. Transparent material is faceted, whereas semitranslucent and translucent material is cut as cabochons. Garnet is one of the few gemstones that does not react to known treatments. Synthetic garnets have not been produced commercially, but are in some cases simulated by glass and even synthetic corundum. Variations in the composition of garnets produce a range of species.

It is found in schists and gneisses. Although brownish-red almandine garnet is known in South Africa, there are no known deposits of gem quality material. Pyrope (Mg3Al2(SiO4)3 ) is often associated with diamonds in kimberlites. The alluvial deposits are generally very small. Gem-quality pyrope pebbles are found in the gravels of Orange and Vaal Rivers. These water-worn stones with their frosted surfaces exhibit a brownish-orange to red brown colour. A misleading name given for these garnets is “Cape ruby”. A purplishpink variety is known as rhodolite or Malaya and is a “mixture” of almandine and pyrope garnet.

This is the yellowish-orange to brownishyellow variety known as hessonite or “cinnamon stone”. Gem-quality transparent material is rare. Orange, yellow and brown spessartine is found in the Pegmatite Valley, Northern Cape Province.

Green and pink hydrogrossular is also known as South African jade or Transvaal jade, because of its jade-like appearance. It has the same chemical composition as grossular garnet, but contains water in the crystal structure. It is massive, translucent to subtranslucent and has a waxy lustre. The colour can be green, due to chromium, or pink, due to manganese. Black, opaque inclusions of magnetite or chromite are present. Massive hydrogrossular also forms a solid solution with the mineral idocrase.

In the finest specimens the material is almost transparent with an intense green colour and the black inclusions are absent. It is cut as cabochons, carvings or jewellery pieces. Hydroglossular is associated with the chromite deposits of the Bushveld Complex. Reserves are reported to be exhausted.

Geological occurrences: In thermally altered aluminium-rich rocks, gneisses, schists, granites, pegmatites and alluvial gravels.

The colours of iolite or “water sapphire”, as known in the trade, may vary from deep blue, violet blue to greyish blue. Iolite has three pleochroic colours and is thus trichroic. The colours range from violet blue to colourless and greyish yellow. Neargem- quality iolite has been reported in the vicinity of Hout Bay, Western Cape Province and Namaqualand. Gem-quality iolite is relatively rare.

Cordilrite (non gem-quality iolite) is present at the contact of the Bushveld Complex and the shales of the Transvaal Supergroup owing to contact metamorphism. Occurrences are reported from Mpumalanga Province, as well as at areas in the Northwest Province. Lenses of massive cordierite occur in slates near the contact of the Cape Granite and rocks of the Cape Supergroup.

Geological occurrences: quartz is a constituent of many igneous, metamorphic and sedimentary rocks. It also occurs in unconsolidated sand and gravel deposits. Quartz is deposited in rocks of all ages and is the most common of all minerals. It is also a very important rock-forming mineral. It may be divided into crystalline and cryptocrystalline quartz.

Clear. These colourless, doubly terminated crystals are also known in the trade as “Herkimer diamond” or “rhinestone”. They occur mainly in granites and pegmatitic veins. It is collected as mineral specimens because excellent crystals are common. At the Messina mines large crystals up to 0.5 m in length have been recovered.

Owes its colourless-to-black (morion) colour to a possible combination of natural irradiation, but the crystal is not radioactive. The large crystals occur in cavities in granite and pegmatite and can be transparent to opaque. Occurrences include those at the Potgietersrus tin mines, Groblersdal district, Barberton district, Namaqualand, Gordonia district and near Xarroies, Northern Cape Province.

The colour varies from deep purple to light violet and is caused primarily by iron oxide. Small amounts of titanium and/or manganese are also present. The colouration is in patches and zones, and rarely uniform. Amethyst is found in geodes (cavities), where the terminations of the crystals are amethyst and the rest often colourless rock crystal. Crystals are transparent to translucent. Gem-quality amethyst is deep purple in colour and is usually facetted. Amethyst is also manufactured synthetically and is cut and used as gemstones, but its price is not much less than that of the natural gemstone. Amethyst is found in the Northern Cape Province, Gordonia district and Gauteng Province (poorquality crystals). Better-quality amethyst is found near Pofadder.

This yellow to golden-brown quartz type is coloured by iron oxide. It is still occasionally called “topaz quartz”, but this is a misleading term that should not be used. The occurrence and habit of citrine are similar to those of amethyst. Although citrine is not very valuable as a gemstone, it is being synthesized on a large scale. Purple amethyst turns yellow (or green) after heat treatment and therefore many heat-treated amethysts are marketed as citrine. Amethyst-citrine quartz (also known as ametrine) shows amethyst (purple) and citrine (yellow) colours together in the same crystal. Quartz is one of the most com - mon minerals, appearing in a vari - ety of beautiful colours, and in an extraordinary perfect hexagonal crystal form. In some cases, one or both colours are artificially induced by heat treatment. Natural ametrine is known, but does not occur in South Africa. Citrine is found at Jagersfontein, Free State Province.

The colour of this variety ranges from from pale to deep pink (“strawberry” quartz) due to traces of titanium in its structure. It occurs massive in pegmatites, but is almost always included and cracked. Distinct crystals are very rare. Rutile-needle inclusions give rise to a bluish appearance, also known as blue quartz. It is used in the trade as beads, pendants or ornaments. Localities include the Northern Province, Namaqualand district, Gordonia district, Kenhardt district, Prieska- Upington area and at Jakkalswater.

This is the general term for finegrained, fibrous quartz. Chalcedony often forms concretionary deposits, usually mamillary, botryoidal or stalactitic. It also occurs as filling in cavities in basalts (igneous rock consisting of minerals plagioclase and augite) and replacements of organic material. The colour ranges from white, through grey and blue to brown and it is transparent to translucent. The various coloured varieties of chalcedony have names such as chrysoprase (green), sard (brown), heliotrope (dark green) and carnelian (reddishbrown). Chalcedony is found near Prieska, Northern Cape Province.

This is distinctly banded chalcedony with successive layers differing in colour. Agate forms as a filling in volcanic rocks (Drakensberg basalts), but is often found concentrated in river gravels. The centre of the cavity is often filled with crystalline quartz or chalcedony or may be empty. The bands are formed by the precipitation of silica, layer by layer, from the outside of the cavity inwards. Often alluvial diamond deposits, such as those in the Orange, Vaal and Caledon Rivers also contain agate. Much of the commercially produced agate is artificially coloured by chemical treatments. All varieties are cut into cabochons, beads, engravings and cameos.

cameos. Lace Agate SiO2: This translucent chalcedony, with blue and white bands, occurs near Springbok, in the Northern Cape Province.

The grey, bluish or milky transparent to translucent chalcedony, where chemical precipitation of iron (red), manganese (black), or chlorite (green) oxide depicts a organic, moss appearance. Poor-quality moss agate is found in the vicinity of Lichtenburg and on the slopes of the Lebombo Mountains, KwaZulu-Natal.

This is banded agate in a limited range of band colours (black and white or grey and white). Onyx is also artificially coloured by chemical treatment.
Occurrences include KwaZulu-Natal Province, Postmasburg, Barkly West, Hopetown, Lichtenburg and Pilgrim’s Rest.

A varicoloured chalcedony in which the presence of iron and manganese contributes to the brilliant colours which include white, yellow, red, brown, blue, grey and black. Banded black and white stones are known as zebra stones. Localities are in the Prieska district.

This is an apple-green chalcedony with spots of chromium-rich muscovite occurring at Gravelotte and Leydsdorp.

An orange to brownish chalcedony found in volcanic rocks and also in alluvial diamonds diggings of Kimberley and Lichtenburg.

This variety is a massive, fine crystalline quartz coloured by iron oxide. Jasper has an opaque appearance. The colour is most often red, but can be
brown, yellow and even green due to chlorite (rare green jasper). Brecciated jasper is also known as zylite. Banded red jasper is layered, with layer thickness varying from a few millimetres to more than 100 mm. Occurrences are restricted to the Postmasburg, Kuruman and Hay districts, Northern Cape Province.

The chemical composition of opal is silica, with up to 10 % water. The structure is non-crystalline and opal consists of very small silica spheres, visible only through a Scanning Electron Microscope. The play of colour is caused by these silica spheres diffracting the light. Opal occurs as a low-temperature mineral in surface deposits. It is also deposited from low-temperature silica-bearing water and may occur in sedimentary, igneous or metamorphic rocks. It may also act as a replacement mineral, e.g. opalised wood. Common opal is semi-translucent to opaque, colourless, white or coloured (impurities). The gem varieties include: White opal with colour play (iridescence), black opal with bright colour effect and fire-opal which is transparent to translucent brownish yellow to orange. Fire opal may be faceted and other varieties are cut as cabochons, carvings and beads. Common opal occurs locally in large quantities in the vicinity of Rustenburg, Gordonia district, and from the Northwest Province usually in the gossan (weathered crusts) of basic rocks or as leaching products of sulphide- rich pipes. No gem opals occur in South Africa

South Africa’s crocidolite and tiger ’s eye deposits are amongst the few in the world that are economically important and mineable. Although the deposits of gem-quality tiger’s eye are very large, they occur only in the banded- iron formations of the northern part of the Northern Cape Province. The crocidolite deposits stretch from Prieska in the south, to Kuruman in the north, a distance of 400 km, and occur in a range of hills known as the Asbestos Hills in the south,and the Kuruman Hills in the north. Crocidolite, or blue asbestos, is a fine, fibrous form of the soda-amphibole riebeckite [NaFe+2Fe+3Si8O22/(OH)2]. The fibrous nature of the crocidolite is not related to dynamic metamorphism, but to low-temperature dehydration and slight ionic rearrangement. In the weathered zone, crocidolite becomes brown and brittle. Under some circumstances the fibres can be replaced by silica while retaining the general outward appearance of the asbestos. This gives rise to the so-called tiger’s eye quartz (93%-97% quartz). Tiger’s eye is yellow and brown in colour. Other variety names include “hawk’s eye” (grey and green), “zebra” (yellow and green/blue), “cat’s eye” (blue - a form of unweathered crocidolite) and “bull’s eye” (red). The last mentioned does not occur in nature, but is obtained by heating tiger’s eye to approximately 400OC Tiger’s eye deposits are found in the vicinity of Niekerkshoop, Prieska and Hay districts and Vryburg district, Northern Cape Province. Reserves have not been quantified, but are large. Mining is by means of open-cast quarries. Tiger’s eye is not manufactured synthetically.