transparent to translucent, natural or synthetic variety of corundum (q.v.; aluminum oxide, Al₂O₃) that has been highly prized as a gemstone since about 800 BC. Its colour is due mainly to the presence of small amounts of iron and titanium and normally ranges from a very pale blue to deep indigo, with the most valued a medium-deep cornflower blue. Colourless, gray, yellow, pale pink, orange, green, violet, and brown varieties of gem corundum also are known as sapphire; red varieties are called ruby. Much sapphire is unevenly coloured; it is also dichroic; that is, the colour of most varieties changes with the direction of view. Alexandrite sapphire appears blue in daylight and reddish or violet in artificial illumination, somewhat like true alexandrite. Careful heating and cooling under various conditions can induce permanent colour changes in sapphire (e.g., from yellow to colourless or greenish blue and from violet to pink). Other colour changes result from exposure to intense radiation. Most sapphire contains abundant microscopic inclusions; reflections from these yield a faint whitish sheen, known as silk. Tiny, regularly arranged mineral inclusions (commonly rutile) and elongate cavities are responsible for the asterism shown by star sapphire.