It was not until the 18th century that it was clearly established that sapphires and rubies are, in fact, the same mineral - corundum, natural aluminum oxide. The name corundum is probably derived from the Sanskrit kuruvinda, meaning "ruby", the name given to red corundum. When it is found in other colors, it is called "sapphire." Next to diamond, corundum is the hardest mineral on Earth. It crystallizes in the hexagonal system, forming dipyramidal or rounded barrel - shapes.
THE COLORS OF SAPPHIRE
Although populary thought of as being blue, sapphire can also be colorless, green, pink, and a wide range of other hues. Rare pink-orange stones are called padparadscha, and sapphire that appears blue in daylight and reddish or violet in artifucal light is called alexandrine or alexandrite sapphire. Sapphires are commonly pleochroic - that is, they appear to be differently colored when viewed from different directions.
All sapphires apart from blue ones are identified by the term "sapphire" preceded by the color. "pink sapphire," "yellow sapphire," and rare, colorless "white sapphire." All colored sapphires other than blue or deep red ones are collectively known as fancy sapphires. Technically, these names are reserved for gem-quality stones, but generally they are simply used for stones of the right color. When blue, sapphire is colored by trace titanium and iron, when pale green, yellow, or brown it is colored by iron, and when pink by very small traces of chromium. With increasing amounts of chromium, pink sapphire forms a continuous color range with ruby. Vanadium nickel, and cobolt can also alter colors. Most corundum contains nearly 1% iron oxide. Much sapphire is unevenly colored, with concentric zones of color; for example, green sapphire is often found to be alternating bands of yellow and blue, but skillful cutting of unevenly colored stones yields gems with a uniform appearance. The color and transparency of sapphire can both be changed by heating it or subjecting it to radiation. The somewhat milky stones from the Missouri River in Montana are clarified by heat treatment, as are many Sri Lankan stones.
