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Rubies

Ruby
Ruby - Winza, Tanzania.jpg
Natural ruby crystals from Winza, Tanzania
General
Category Mineral variety
Formula
(repeating unit)
aluminium oxide with chromium, Al2O3:Cr
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space group R3c
Identification
Color Near colorless through pink through all shades of red to a deep crimson
Crystal habit Terminated tabular hexagonal prisms.
Cleavage No true cleavage
Fracture Conchoidal, splintery
Tenacity Brittle
Mohs scale hardness 9.0
Luster Subadamantine, vitreous, pearly (on partings)
Streak white
Diaphaneity transparent, translucent
Specific gravity 3.97 – 4.05
Optical properties Uniaxial -
Refractive index nω=1.768–1.772
nε=1.760–1.763
Birefringence 0.008
Pleochroism Strong: purplish-red – orangy-red
Dispersion 0.018
Ultraviolet fluorescence red under longwave
References

A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. Ruby is considered one of the four precious stones, together with sapphire, emerald and diamond.

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 "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 more pink than garnet, although some rhodolite garnets have a similar pinkish hue to most rubies. The world's most expensive ruby is the Sunrise Ruby.

Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Among the natural gems only moissanite and diamond are harder, with diamond having a Mohs hardness of 10.0 and moissanite falling somewhere in between corundum (ruby) and diamond in hardness. Sapphire, ruby, and pure corundum are α-alumina, the most stable form of Al2O3, in which 3 electrons leave each aluminum ion to join the regular octahedral group of six nearby O2− ions; in pure corundum this leaves all of the aluminum ions with a very stable configuration of no unpaired electrons or unfilled energy levels, and the crystal is perfectly colorless.


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Wikipedia

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