Birthstone: September
Moh's hardness: 9
Specific Gravity: 3.95-4.03
Color: Blue in various tones, colorless, pink, orange, yellow, green, purple, black
Chemical Composition: Al2O3 aluminum oxide
Refractive Index: 1.762-1.788

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The name sapphire {greek means blue} used to be applied to various stones.  In antiquity and as late as the Middle Ages, the name sapphire was understood to mean what is today described as lapis lazuli.   Around 1800 it was recognized that sapphire and ruby are gem varieties of corundum.  At first the blue variety was called sapphire and corundums of other colors {with the exception of red} were given special, misleading names, such as "oriental peridot" for the green variety or "oriental topaz" for the yellow type. 

Today corundums of gemstone quality of all colors except red are called sapphire.  Red varieties are called rubies.  The various colors of sapphire are qualified by description, e.g. green sapphire or yellow sapphire.  Colorless sapphire is called leuko-sapphire {greek means white}, pinkish orange sapphire Padparadscha. 

There is no definite demarcation between ruby and sapphire.  Light red, pink or violet corundums are usually called sapphires, as in this way they have individual values in comparison with other colors.  If they were grouped as rubies, they would be stones of inferior quality.   The coloring agents in blue sapphire are iron and titanium; and in violet stones, vanadium.  A small iron content results in yellow and green tones; chromium produces pink, iron and vanadium orange tones.  The most desired color is a pure cornflower-blue.  In artificial incandescent light, some sapphires can appear to be ink-colored or black-blue. 

Through heat treatment at temperatures of about 3100-3300 degrees F {1700-1800 degrees C}, some cloudy sapphires, non distinct in color, can change to a bright blue permanent color.

Hardness is the] same as ruby and also differs clearly in different directions {an important factor in cutting.  There is no fluorescence characteristics for all sapphires.  Inclusions of rutile needles result in a silky shine; oriented, i.e. aligned, needles cause a six-rayed star sapphire.