RUBY AND SAPPHIRE are varieties of the mineral corundum, an aluminum 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 wear.
It is pleochroic, which means the colour of a 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 countries in East Africa.
BERYLS ARE POPULAR AS GEMS for their fine colours and resistance to wear. The most well-known varieties are emerald (green) and aquamarine (blue green). Yellow beryl is known as heliodor, and pink beryl is morganite. The name beryl can be traced to Greek, Roman and possibly sources, and it is highly likely that aquamarine heliodor were known in prehistoric times. Beryl is found in pegmatites and granites and in its massive, opaque, non-gem form can occur crystals weighing many tonnes. The record holder is a crystal which was found in Madagascar weighing 36 tonnes and measuring about 18 m (60 ft) long.
THE POPULARITY OF OPAL has risen and fallen over the centuries. The Ancient Romans used it as a symbol of power but at different times since then, it has been considered to be unlucky. The Aztecs mined opal over 500 years ago in Central America which is still an important source, especially of fire opal.
The top producer of black and white opal is Australia where it was first discovered during the 1870s. Opal is one of the few non-crystalline gems. It has a tendency to crack and chip especially due to extreme temperature changes or a hard knock. The exciting flashes of colour shown by precious opal are best displayed in cabochons although Mexican fire opals are usually cut as brilliants or step-cuts.