As mentioned earlier, sapphire is corundum, too, and, its physical and chemical properties are virtually identical to those of ruby. In a natural state the gem crystallizes in the hexagonal system with two pyramidal faces. It is fractionally harder than ruby. Curiously, all corundums that are not rubies are classed as sapphires, which means that this particular gem exists in many colours, from blue to green, pink to mauve, white, grey, violet, yellow and orange.

The blue colour of sapphire is due to small amounts of titanium oxides and iron. The finest colour is found in the Kashmir specimens. Ceylon sapphires are generally of a vivid blue, sometimes only partially coloured. Those found in the United States (Montana) have a bright metallic sheen, while Australian sapphires are of a deep blue-green, almost black even. These are the least valuable but most widespread, being mined in great quantity. The coloured sapphires popular today come mainly from the mines in Thailand, Ceylon and Australia. Multi-coloured specimens are also known to exist.

Transparent sapphire, too, cut en cabochon, reflects the light to produce star-like bands. These star stones range in colour from black (black stone sapphire) to blue (saphir etoile). There is also a variety known as alexandrite which is blue in daylight and red-violet in Artificial light.

Both sapphires and rubies have been successfully and widely produced synthetically by the chemist, and in appearance, chemical composition and hardness are almost identical to the natural gems. Manufactured gems can generally be detected by the presence of minute black-rimmed, bright-centered air bubbles or curved bands, visible with the aid of a l0x magnifier.

The focus of the synthetic corundum production is in France, and South East Asian countries are among the biggest buyers. So beware of buying stones from foreigners or well-meaning friends who have just returned from an oriental holiday with the "bargain of the year" Your best course of action is to consult a professional jeweler of sound reputation. In all events, seek expert advice.