According to the 11th-Century French bishop Marbodius, "ruby is the solitary and glowing eye which dragons and wyverns carry in the middle of their foreheads." In less poetic modern terms, ruby is the red variety of the mineral corundum, an oxide of aluminum. It ranges in color from deep cochineal to pale rose-red, sometimes with a tinge of purple; the most valued is a blood-red. Its coloration comes from traces of chromium (as small a 1 part in 5,000) which replaces some of the aluminum in the structure. Ruby forms a continuous color succession with pink sapphire, the color deepening with an increase in chromium. Only stones of the darker hues are generally considered to be ruby. It was believed by ancient Hindu and Burmese miners that colorless or pale pink sapphires were rubies that had not completely ripened. Ruby has been mined from the gem gravels of Sri Lanka since the 8th century BC.
KING OF PRECIOUS STONES
The name ruby comes from the Latin ruber, meaning "red." In Sanskrit, ruby is known as ratnaraj, "king of precious stones."
Many rubies are heat-treated to imporve their clarity or color (or both). Rubies tend to be small (stones more than 10 carats are unusual) as the presence of chromium has an inhibiting effect of crystal growth- hence the high value of large rubies. Ruby crystal tend to be haxagonal prisms with tapering or flat ends. They occur worldwide in igneous and metamorphic rocks, or as waterworn pebbles in alluvial deposits. Gem-quality ruby is a mineral of much more limited distribution. It is found in the following localities: in northcentral Myanmar, where it occurs in bands of crystalline limestone; in gravels in Thailand with sapphires and spinels; and in the gem gravels of Sri Lanka. Other localities that produce minor amounts of gem material are Afghanistan, Madagascar, Vietnam, Cambodia, and North Carolina and Montana. Bright red, opaque ruby crystals in green zoisite come from Tanzania. Star rubies occur in addition to star sapphires.
