July Birthstone - Ruby

Physical and Chemical Properties

Chemical composition

Rubies are a variety of the mineral corundum, aluminum oxide (Al2O3). Ruby and its companion sapphire, are very hard---9 on Mohs scale of 10.

Only diamonds and a few manufactured abrasives such as boron carbide and silicon carbide are harder. Rubies are fairly dense---the specific gravity ranges from 3.95 to 4.10 but most is almost exactly 4.00. Since ruby crystallizes in the hexagonal system it has two refractive indexes. These range from the extraordinary ray (the one that varies) of 1.762 on the low end to the ordinary ray (the one that remains fixed) of 1.770 on the high end. Ruby is said to be uniaxial negative. Rubies may be dichroic

Colors

Color is the most important character of a ruby when it comes to a jeweler properly representing the stone. All rubies must be shades of red, orange- red, or purple red. There is no such thing as a pink ruby. By definition pink corundum is a sapphire. Rubies can vary in hue from brilliant red to violet or brownish red. Any non-red variety of corundum is considered sapphire.

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