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Ruby Mineral Facts:

Chemical Formula: Al₂O₃
The mineral is nearly always a practically pure oxide of aluminum of the composition Al₂O₃, in which there are 52.9 per cent Al and 47.1 percent O. The impure varieties usually contain some iron, mainly as an admixture in the form of magnetite.

Colors: Ruby is always some shade of Red. Other Corundum colors are known as sapphire.

Hardness: 9.0
Next to the diamond in hardness.

Density: 3.95 to 4.1
The density is unusually high for a nonmetallic mineral. 

Cleavage: None, but separation planes parallel to the basal plane common. Parting basal and rhombohedral, the latter giving nearly cubic blocks.

Crystallography: Hexagonal-Rhombohedral
Crystals usually prismatic in habit or tapering hexagonal pyramids.

Luster:. Adamantine to vitreous luster. Transparent to opaque.

Optics: (Refractive Index):  w= 1.7690; e= 1.7598

Composition, Structure and Associated Minerals:
Corundum is common in the metamorphic rocks, such as crystalline limestone, mica-schist, gneiss, etc. Found also as an original constituent of certain igneous rocks, usually those deficient in silica. Found sometimes in large masses, evidently the product of magmatic differentiation. Found frequently in crystals and rolled pebbles in detrital soil and stream sands, where it has been preserved through its hardness. Associated minerals are commonly chlorite micas, chrysolite, serpentine, magnetite, spinel, cyanite, diaspore, etc. Also occurs as the result of the contact metamorphism of limestones, as in Burma. The corundum gem-stones occur either as isolated crystals in limestone, etc., or as rounded pebbles in alluvial deposits.

Corundum is the hardest mineral known, with the exception of diamond. In consequence of its great hardness an impure variety is used as an abrading agent under the name of emery. It is also one of the most valuable of the gem minerals. It occurs as crystals and in granular masses. When transparent and finely colored, corundum furnishes various gem stones. The ruby is deep red corundum; sapphire is a corundum of colors other than red.

Identification and Diagnostics
Infusible and insoluble. Finely pulverized material moistened with cobalt nitrate and intensely ignited assumes a blue color (a test for aluminum). Characterized chiefly by its red color, great hardness, adamantine luster and high specific gravity.

Occurrence, Localities and Origins:
Used as a gem stone. Very high quality ruby yields the most valuable of gems; a stone of the deep red known as "pigeon's blood" may bring many hundreds of thousands of dollars a carat.

Emery was formerly used also as an abrasive, but the majority of such uses are replaced either by silicon carbide or synthetic corundum.

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For More information about Ruby as a gem, see:  
RUBY - The Fiery Red Gem

Nevada Outback Gems