In the introduction to gemstones we covered the four classic precious stones namely: diamonds, emeralds, rubies and sapphires. Here we look at some less well-known gemstones for example chrysoberyl, or another which has rather lost its appeal to the public - at least for the time being. I am referring of course to zircon.
The hardness of chysoberyl is 8.5 which makes the stone one of the hardest after diamond. Chrysoberyl occurs in granitic rocks, pegmatites and mica schists; is often found in alluvial deposits. It has also been found in contact metamorphic deposits of dolomitic marble with corundum, and in fluorine skarns. Most chrysoberyl is recovered from river sands and gravels. Most chrysoberyl comes from Sri Lanka, southern India, Brazil (Minas Gera is) and Burma.
The Egyptians claimed that topaz was coloured with the golden glow of the mighty sun god Ra. This made topaz a very powerful amulet that protected the faithful against harm. The Romans associated topaz with Jupiter, who also is the god of the sun. However, the stone they referred to may have actually been chrysolite or peridot and it is very possible that the topaz of modern mineralogy was unknown to the ancients. In 1750 a Parisian jeweller discovered that the yellow Brazilian topaz becomes pink on exposure to a moderate heat, and this treatment has since been used extensively, so that nearly all the pink topaz in jewellery has been heat-treated. Such "burnt topaz" is often known as Brazilian ruby, as is the very rare, natural red topaz.
Topaz typically occurs in cavities in rhyolites and granite, in pegmatite dikes, and in high-temperature veins with cassiterite and tourmaline. It is one of few gem minerals which, under suitable conditions, might grow into huge crystals reaching several hundred thousand kilograms in weight! Important sources of topaz are in Russia, Brazil, Sri Lanka, Africa, China, Japan, Pakistan, Myanmar, Nigeria, Australia, Mexico, and in the United States (in Maine, New Hampshire, California, Colorado, and Utah). In the United States the best topaz has been found near Pikes Peak, Colorado, and in San Diego county, California. The largest known deposits are located in Minas Gerais in Brazil. The finest British topaz is found in the Cairngorm Mountains in the Central Highlands, especially at Ben a Buird, Scotland. The famous topaz rock of the Schneckenstein, in Germany, yields pale yellow crystals.
Zircon has been known for hundreds of years, the mineral and its varieties even being mentioned in the bible. Despite this, zircon is out of favour these days - a most unfashionable gem. There are number of reasons for this. Zirconium (Zr) is not particularly rare. It is the 17th most abundant element on earth, and as such, is more plentiful than copper, lead or tin. Although it comes in a variety of colours, none of them are exeptional. The gems are quite brittle and easy to chip and scratch and so require special care. But zircon has its claims to fame. It is among the oldest rocks on earth, formed over 4.3 billion years ago. The legend of Zircon began when Hyacin, a youth in Greek mythology was killed and a blue hyacinth flower grew from his spilled blood. The blue zircon found in Greece matched the blue of the flower. Pliny the Elder linked the stone with this legend in his essay on the comparison of colours.
Zircon is widespread as an accessory mineral in acid igneous rocks, and also occurs in metamorphic rocks. It is a component in the beach sands of many parts of the world, particularly Australia, India, Brazil, and Florida. Gem varieties are often found in stream gravels and detrital deposits, particularly in Indochina and Sri Lanka, but also in Burma, Australia, and New Zealand. Zircon forms an important part of the syenite of southern Norway and occurs in large crystals in Quebec. Zircon is also found in Cambodia, France, Myanmar, Thailand, Nigeria and Tanzania.
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