"The blue brilliance of the sapphire has been treasured for thousands of years, its color thought to represent heavenly or celestial qualities. The ancient Persians believed that the earth rested on a giant sapphire and that its reflection gave the sky its blue color. Jewish tradition holds that Moses was given the Ten Commandments on tablets of sapphire, making it the most sacred of gemstones.
Sapphire is a form of the mineral corundum, a crystalline form of aluminum oxide and one of the most durable minerals that exists; only diamonds are harder. Trace impurities of iron and titanium are responsible for the deep blue color most people associate with sapphire. The gem also occurs in a variety of blue shades. Several other colors of corundum, such as yellow, reddish-orange, and violet, are also classified as sapphire. Red corundum crystals are called rubies. When cut into a cabochon (a convex, unfaceted form), some specimens of sapphire exhibit asterism; that is, a six-rayed star can be seen in the interior of the stone. Such stones are called star sapphires."