Ruby is a light pink to blood red gemstone, a variety of the mineral corundum (aluminium oxide). The common red color is caused mainly by the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. It is considered one of the four precious stones, together with the sapphire, the emerald and the diamond. Improvements used include color alteration, improving transparency by dissolving rutile inclusions, healing of fractures (cracks) or even completely filling them.
Prices of rubies are primarily determined by color (the brightest and best "red" called Pigeon Blood Red, command a huge premium over other rubies of similar quality). After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions will indicate the stone has been treated one way or another. Cut and carat (size) also determine the price.
more about ruby : http://en.wikipedia.org/wiki/Ruby
Ruby crystal before faceting, length 0.8 inches (2 cm)
|Chemical formula||aluminium oxide with chromium, Al2O3::Cr|
|Color||Red, may be brownish or purplish|
|Crystal habit||Varies with locality. Terminated tabular hexagonal prisms.|
|Cleavage||No true cleavage|
|Fracture||Uneven or conchoidal|
|Mohs Scale hardness||9.0|
|Pleochroism||Orangey Red, Purplish Red|
|Ultraviolet fluorescence||red under longwave|