A family of minerals including the ruby, the padparadscha, and the sapphire (any gemstone-grade corundum that isn't red like a ruby or orange like a padparadscha is called a sapphire). They are second only to diamonds in their hardness (see the Mohs hardness scale) and thus are very durable in jewelry. Asterism is also more common in corundum than other gems.

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.

Co*run"dum (k?-r?n"d?m), n.; pl. Corundums (-dmz). [Also corindon.] [From Hind. kurand corundum stone.] Min.

The earth alumina, as found native in a crystalline state, including sapphire, which is the fine blue variety; the oriental ruby, or red sapphire; the oriental amethyst, or purple sapphire; and adamantine spar, the hair-brown variety. It is the hardest substance found native, next to the diamond.

⇒ The name corundum is sometimes restricted to the non-transparent or coarser kinds. Emery is a dark-colored granular variety, usually admixed with magnetic iron ore.

 

© Webster 1913.

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