Variety of corundum, mineral, gemstone / From Wikipedia, the free encyclopedia

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A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapphires. Ruby is one of the traditional cardinal gems, alongside amethyst, sapphire, emerald, and diamond.[3] The word ruby comes from ruber, Latin for red. The color of a ruby is due to the element chromium.

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A ruby crystal from Dodoma Region, Tanzania
CategoryOxide mineral variety
(repeating unit)
aluminium oxide with chromium, Al2O3:Cr
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space groupR3c[1]
ColourOrangy red through strongly purplish red.
Crystal habitTerminated tabular hexagonal prisms
CleavageNo true cleavage
FractureConchoidal, splintery
Mohs scale hardness9.0
LusterSubadamantine, vitreous, pearly (on partings)
DiaphaneityTransparent, translucent
Specific gravity3.97–4.05
Optical propertiesUniaxial/−
Refractive indexnω=1.768–1.772
Birefringence0.008 to 0.010
PleochroismStrong: purplish-red – orangy-red
Ultraviolet fluorescenceRed under longwave
Common impuritiesCr. (sometimes :Ti, Fe)
Main ruby producing countries

Some gemstones that are popularly or historically called rubies, such as the Black Prince's Ruby in the British Imperial State Crown, are actually spinels. These were once known as "Balas rubies".

The quality of a ruby is determined by its color, cut, and clarity, which, along with carat weight, affect its value. The brightest and most valuable shade of red, called blood-red or pigeon blood, commands a large 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 may indicate that the stone has been treated. Ruby is the traditional birthstone for July and is usually pinker than garnet, although some rhodolite garnets have a similar pinkish hue to most rubies. The world's most valuable ruby to be sold at auction is the Sunrise Ruby.