Gemstones Part II


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.

cat's eyeChrysoberyl     BeAl2O4
                The name chrysoberyl comes from the Greek: chrysos meaning 'golden', plus 'beryl' which is from the Greek berrulos, meaning 'crystal', so chrysoberyl = golden crystal. But chrysoberyl is not the same as beryl, which is a different precious stone altogether! Chrysoberyl is among the world's rarest gems. A beautiful specimen of chrysoberyl (47 carats) can be seen in the British Museum of Natural History, but the largest cut stone is in the Smithsonian (66 carats). One of the characteristic features of some varieties of chrysoberyl (for example from the Ural mountains in Russia) is their so-called "Alexandrite Effect" where the stone looks green in daylight (rich in blue light) and red in candlelight (rich in red light). This variety is commonly known as alexandriteas and it was discovered in 1831. The "Alexandrite Effect" is particularly obvious in thicker stones and those who specialize in mineral chemistry will tell you that the effect is mediated by the substitution of chromium for aluminium kations. Another variety of chrysoberyl - chrysolite has a light greenish-yellow colour. Finally, the translucent yellowish variety is called cymophane from greek meaning "wave", but is better known as "cat's eye". (An example can be seen above.) The effect of a single ray of light passing across the crystal is achieved by microscopic tubelike cavities or needlelike inclusions of rutile inside the stone.

                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.

topazTopaz     Al2F2SiO4
                There is still some uncertinty as to the origin of the name. One source states that the name topaz is derived from the Indian Sanskrit word tapas, meaning fire. According to another theory, topaz derives its name from the Island of Topazos, in the Red Sea, where the Romans obtained a stone which they called by this name. Pure topaz is colourless, but it can also occur in a range of colours: yellow, blue, pink, peach, gold, green, red, and brown. The pure crystals of topaz are used a great deal in jewelry. The most sought after forms of topaz are the light yellow, brown and pink varieties.

                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.

zirconZircon    ZrSiO4
                The name Zircon is originated from Persian Zargun which means "gold colour" and it comes in three main varieties:

  1. the colourless variety is known as Ceylon or Matura diamond because it is used to imitate diamonds. The high refractive index and dispersion of zircon cause it to come close to diamonds in fire and brilliancy.
  2. the orange variety of zircon is called jacinth. Jacinth was a very popular stone in classical antiquity
  3. the blue variety is called starlite or Siam Zircon

                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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