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Diamond The name diamond is derived from the Greek word adamas meaning ‘unconquerable’, given to the stone because of its supreme hardness. Diamond is made of pure carbon and has an immensely strong crystal structure. It is this which makes it hardest of all minerals. Evidence suggests that diamonds were formed up to 200 km (125 miles) deep within the Earth, and some stones may be as much as 3,000 million years old. Diamonds were first discovered over 2,000 years ago and came mainly from river gravels in India. In 1725, they were found in Brazil which was the major source until production in South Africa became significant in 1870. Today, about 20 countries produce diamonds. The top producer is Australia, which supplies a quarter of the world’s needs. |
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Corundum Ruby and sapphire are varieties of the mineral corundum, an aluminium oxide. Only true red stones are called rubies and the term sapphire on its own indicates a blue stone. Other colours are described as sapphire, e.g. yellow sapphire and pink sapphire. Corundum is next to diamond in hardness, so gem crystals are resistant to ware. It is pleochoric, which means that colour of stone varies when it is viewed in different directions. Most gem crystals are recovered from gravels and the most famous sources are Myanmar, Kashmir and Sri Lanka. Today, Australia is the largest producer of blue and golden sapphires. Other producers include Thailand and counties in east Africa. |
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Topaz The mineral we know as topaz was only given the name in the first half of the 18th century. Prior to that, its history is not clear. The name Topaz is said to come from Topazius, the Greek name for Zabargad, an island in the Red Sea. This island is however source of what we now call Peridot. Hardness 8, specific gravity 3.52 – 3.56. |
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Zircon The name zircon comes from the Arabic zargoon meaning vermillion or golden coloured. Sri Lanka has been a source of zircons for 2,000 years but today stones also come from Thailand, Australia and Brazil. Colourless zircon looks like diamond in lustre and fire and is used as a diamond stimulant, but it is softer and may look ‘sleepy’ due to inclusions and double refraction. |
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Turquoise The name turquoise comes from French Pierre Turquoise, meaning stone of Turkey, most of it in past being sold in Turkey. It occurs in nodules and veins of green or blue. Copper makes it blue; iron makes it green. It has a specific gravity 2.6 – 2.9 and the hardness of 5 – 6. Turquoise is rare in large masses and is more often found forming mosaics. The finest blue turquoise comes from Iran (previously called Persia), where it has been used for decoration for almost 6,000 years. |
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Lapis Lazuli Lapis Lazuli is not a single mineral, but rock consisting of blue lazurite with variable amounts of calcite and pyrite. The best, from Afghanistan, consists mostly of lazurite and is deep blue. It is 5.5 on Moh’s hardness scale and has a specific gravity of 2.7 – 2.9. There are other sources in Russia and Chile. In medieval times, lapis lazuli was crushed and purified to make the paint pigment ultramarine. It was used to paint the Wilton diptych, a detail of which is shown here. The famous altar piece is now in National Gallery in London. |
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Coral Coral is a skeleton of calcium carbonate made by colonies of soft-bodied animals which live in tropical or sub-tropical waters. The range of colours, from black to blue to cream to red, is due to different growth conditions and organic contents. |
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Pearl and shell |
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