THE PROPERTIES OF A WONDERFUL GEMSTONES are said to be beauty, rarity, and durability and these criteria have been applied to many species. As well as quartz, diamond, ruby and sapphire, beryl, and opal, gemstones to be seen in jewellers' shops include topaz, tourmaline, garnet, Peridot, and many others. Some species such as kunzite, sphene, and fluorite are too soft or rare to be in general circulation as gemstones and are cut only for collectors on the basis of their beauty and rarity.
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 Topaz us, the Greek name for Zabargad, an island in the Red Sea. This island is however a source of what we now call Peridot.
Tourmaline is a mineral with a complex chemistry crystallizing as prisms with flat or wedge-shaped terminations. Every crystal has a different structure at each end, sometimes indicated by different colours. This gives tourmaline an unusual electrical property.
If a crystal is gently warmed, one end becomes positively charged and the other negatively charged, which is the reason for its tendency to attract dust.
Garnet is the name of a family of chemically related minerals that includes almandine, pyrope, spessartine, grossular, and andradite. They can all be found as gemstones with the almandine-pyrope group being the most widely used. Because of the different chemical compositions, garnet occurs in most colours other than blue. Sources of gem-quality material include Czechoslovakia, South Africa, USA, Australia, Brazil, and Sri Lanka.
Peridot is a French word and may derive from the Arabic faridat meaning a gem. It is the gem variety of the mineral olivine, a magnesium and iron silicate, which is common in volcanic rocks.
Moonstone is the best-known feldspar gem. Feldspars are common in rocks, but are rarely of gem quality. There are two main i groups: one which is rich in potash and includes the moonstones; one which is rich in soda and calcium and includes sunstone. They range in hardness from 6-6.5 and in specific gravity from 2.56-2.76.
The most beautiful red and blue spinels can rival ruby and sapphire in their richness. Until the 19th century, it red spinets were called balas rubies which led to some confusion. The scientist Rome de l'Isle was the first to distinguish clearly between true ruby and red spine]. The term balas may relate to a source of these stones in Balascia, now called Badakhshan, in Afghanistan.
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.
Gem Chrysoberyl is exceeded in hardness only by diamond and corundum. The yellow, green, and brown colours are caused by iron or chromium. There are three varieties: clear yellow-green gems; cat's-eye or cymophane, usually cut as cabochons to display the "eye" effect; alexandrite, famous for its dramatic colour change. Sri Lanka and Brazil are sources for all three but the best alexandrites come from the USSR.