Birthstone: July
Moh's hardness: 9
Specific Gravity: 3.97-4.05
Color: Varying red        
Chemical Composition: Al2O3 aluminum oxide
Refractive Index: 1.762-1.778

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Two color varieties of curndum are used for making jewelry, the red ruby and the sapphire.  Common corundums i.e. those not of gemstone quality serve as cutting and polishing material.  The well-known polishing material emery is mainly fine-grain corundum, to which magnetite, hematite and quartz are added.  The name corundum has its origin in India and probably referred to ruby.

Ruby is named because of its red color.  It was not until about 1800 that ruby, as well as sapphire, was recognized as belonging to the corundum species.  Before that date, red spinel and the red garnet were also designated as ruby. 

The red color varies within each individual deposit, so it is not possible to determine the source area from the color.  The designation "Burma ruby" or Siam ruby" are therefore strictly erroneous and refer more to quality than origin.  The most desirable color is the so-called "pigeons blood", pure red with a hint of blue.  The distribution of color is often uneven, in stripes and spots.  The substance that provides the color is chromium and in the case of brownish tones, iron is present as well.  AS a rough stone ruby appears dull and greasy but when cut the luster can approach that of a diamond.  Heat treatment is commonly used to improve the color.

Ruby is the hardest material after diamond.  Inclusions are common.  They are not always indicative of lower quality, but show the difference between natural and synthetic stones.  The type of inclusion {minerals, growth structures, canals, or other cavaties} often indicates the source area.