Corundum: Rubies and Sapphires
Rubies and Sapphires are two gemstone varieties of the mineral, corundum. Corundum is a crystalline form of aluminum oxide. If the corundum is composed entirely of aluminum oxide, it is colorless.
Ruby owes its color to very small amounts of chromic oxide in the lattice. The blue color in sapphire is caused by the presence of titanium and iron. Although all rubies are red, not all sapphires are blue. Sapphires come in a variety of appealing colors including green, yellow, lavender, salmon, and pink. Orange sapphires are called “padparadschas,” a name which alludes to the color of the lotus flower.
A six rayed star phenomenon knows “asterism” is exhibited by some corundums when cut as a cabochon. The Black Star of Queensland, weighing 733 carats is the largest gem-quality star sapphire in the world. Discovered in Australia in the 1930’s, its inky body color is a perfect foil for its luminescent star.
Rubies and sapphires are found all over the world. Important sources are Thailand, Myanmar, Tanzania and Madagascar. “Kashmir” sapphires from the northwestern Himalayas are renowned as some of the finest blue sapphires ever discovered. Lively, attractive blue sapphire has been found in Montana’s Yogo Gulch area. Generally these crystals are rather small, and will only yield gems of sizes less than one carat.
Corundums are suitable for every day wear, rating as 9 on the Mohs hardness scale, second only to diamond. Some corrundums show a definite change in hue from natural light to artificial light. This color change phenomenon occurs when transition metals, such as vanadium and chromium, are present in the crystal.
The faceting of corundum requires great skill. Color is generally uneven in the rough material. The cutter must orient the desirable color areas properly for the finished gem to achieve its maximum potential. Typically the area of most intense color is positioned at the culet of the stone, so that it can radiate up through the finished gem.