Physical Properties of CorundumChemical Classification: Oxide
Color: A gem corundum with a dominant red color is a ruby. Any other color of corundum is a sapphire.
Streak: Colorless (harder than the streak plate)
Luster: Adamantine to vitreous
Diaphaneity: Transparent to translucent
Cleavage: None. Corundum does display parting perpendicular to the c-axis.
Mohs Hardness: 9
Specific Gravity: 3.9 to 4.1 (very high for a nonmetallic mineral)
Diagnostic Properties: Hardness, high specific gravity, six-sided crystals sometimes tapering to a pyramid, parting, luster, conchoidal fracture
Chemical Composition: Al2O3
Crystal System: Trigonal
Uses: Historically used as an abrasive. Specimens with pleasing colors have a long history of gemstone use.
Most people don't realize that ruby and sapphire are both gems of the mineral corundum. Both of these gemstones have the same chemical composition and the same mineral structure. Trace amounts of impurities determine if a gem corundum will be a brilliant red ruby or a beautiful blue sapphire. It is surprising that "impurities" can produce such wonderful results!
Red and blue are just two of the many colors found in gem corundums. Trace amounts of other elements can produce brilliant yellow, orange, green, and purple gems.
Red corundums are known as "rubies," blue corundums are known as "sapphires," and corundums of any other color are known as "fancy sapphires."
Impurities cause corundum to occur in a spectrum of colors, and when it is completely free of color-causing impurities, it is a colorless gem known as "white sapphire."
Very few specimens of corundum have a natural color within the range required for a ruby. Very few also have the clarity required to produce a nice faceted stone. Long ago, people who prepared gem materials for cutting began experimenting with ways to improve their color and clarity.
Heating corundum crystals under controlled conditions can improve or intensify their color. Heating can also remove inclusions by causing them to dissolve, making them less visible and improving the clarity of a gem.
Most rubies in the market today have been heated to improve their color and clarity. This heat treatment is normal and expected in the gem trade, but a seller should disclose the treatment to a buyer in advance of a sale.