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Gifts from the Earth: Sapphires

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The word sapphire comes from the Greek "saapherios", which means blue. These are the blue gemstones most often seen in jewelry, with colors ranging from pale, ice blue to deep, cobalt blue, but did you know that sapphires also come in other colors? Blue is just the more highly prized variant of the mineral corundum. Sapphires also come in orange-pink, golden, white, and even black. The natural crystal form of sapphire is called a trigonal (three-sided prisms), examples of which are in the photo below, at right. The photo on the previous page, contains all sapphires and the various colors in which they are found. Some more interesting varieties of sapphire with rutile (the ore form of titanium)intrusions cause light to be reflected in a six-point star, creating what is known as a star sapphire. This star shaped effect is known as an asterism and is a less pure form of sapphire. All colors of the translucent form of the mineral corundum are known as sapphires. If, however, there are just the right quantities of chromium in the corundum a deep red ruby is formed...

Rubies

Only the red-colored forms of the mineral corundum are called rubies. All other colors are sapphires, preceded by the word "fancy" or by the special color name if they are anything other than blue. The natural crystal form of an uncut ruby is a hexagonal prism or blade (see below right). The largest ruby in the world is the Eminent Star ruby, believed to have come from India. It is 6,465 carats.

Rubies are in a class by themselves. They are the rarest of the most precious gemstones in the world and a large (5 carats or more), translucent ruby can fetch millions of dollars. Both rubies and sapphire specimens that have impurities, or have weaker colors can be enhanced by heat treating them. This helps to remove the impurities that cloud the stones and deepens their color. Heat treated-stones are not as desirable as those that are naturally translucent and deeply colored.