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Corundum
Pure corundum is colorless; the color of sapphire results because of impurities. Corundum is a sort of aluminum oxide. It has no cleavage, which means it doesn't split apart along crystalline planes and it doesn't melt easily. The melting point of corundum is around 3,686 degrees F. It's also insoluble, which means it doesn't dissolve in a medium, like seawater, that would make other minerals dissolve. Corundum is found in marine or alluvial sand, often with mica schists. Some of the crystals found have been huge, up to 375 lbs.
Sapphire's Optical Illusion
The sapphire crystal is barrel shaped or pyramidal and is highly pleochroic. This means it has a quality that fools the eye into thinking that it's different colors when it's observed at different angles. This optical illusion requires the stone to be cut carefully. Sapphire can be found in any color save red; if it's red it's a ruby, and all other non-blue sapphires are called "fancies." Sapphire is found in many countries including Australia, Thailand, Sri Lanka, Madagascar and South Africa. The best sapphires are a brilliant, velvety violet blue and are most often found in Kashmir.
Impurities
The impurities that make sapphires blue are titanium and iron. Sometimes when the sapphire has an inclusion of rutile, a mineral made up mostly of titanium dioxide, it will cause a light like a star in the gem, which is called an asterism. Star sapphires are quite rare and much sought after. Iron and titanium give sapphire its blue color thanks to an electronic interplay between the iron, titanium and aluminum oxide called intervalence charge transfer. Chromium causes the red color in rubies.
Iron
Iron is an abundant metal found in the Earth's crust, the sun, oceans, streams and in human hemoglobin. It's not usually found in its pure state, but is more often alloyed with other metals. In its pure state it's a gray and lustrous metal, but corrodes easily. It's also magnetic.
Titanium
Titanium is also fairly common and in its pure form is a lustrous white metal. Like iron, it's found in the earth, oceans, streams and generally around the universe. Unlike iron it resists corrosion but burns in air and is the only element that burns in nitrogen. It's fairly insoluble, especially in seawater. Unlike iron, it's nonmagnetic.