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Corundum is a trigonal crystal with a chemical base of Al2O3. It has a hardness of 9, and is the hardest of the stones that is measured using the Mohs scale in a fairly linear fashion, whereas diamond, which is a 10, is many orders of magnitude harder. Corundum has a density of approximately 4 grams/cc, making it one of the heaviest among the exceptionally hard stones. It is also a birefringent stone, having indices of refraction of 1.76 and 1.77.

Corundum comes in many shades, including red, orange, yellow, green, blue, violet, grey, pink, black, colorless, and any blend in between, and range from transparent, through translucency, to opaque. If they are sufficiently red, they are called rubies (or carbuncle) and any other coloration is referred to as a shaded Sapphire, with blue being the most recognized. The one exception to this is asteriae, in which rutile (Titanium Oxide, or TiO2) is no longer soluble within the crystal and forms needle-like inclusions within the corundum, along the planes of trigonal reflection. This phenomena causes a perfect six-pointed star reflection to be formed within the crystal whenever an approximately point source of light is used, and will cause one reflection for each source used (within reason of detection by optical means). Aesteriae are generally found in Burma and Sri Lanka, though occasionally in Australia and India as well.