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Physical Properties
Onyx is part of the chalcedony family, which is part of the quartz group. It is associated with carnelian, agate and bloodstone. On the Mohs scale of hardness, which goes from 1 to 10, onyx rates at a 6.5 to 7 and has a density of 2.58 to 2.65. The formula for onyx is SiO2.
Color
Onyx can be white, reddish brown, brown or multilayered black and white stone. Usually an agate or a chalcedony is dyed to make an onyx stone solid black in color. To make the black more intense, it is heat treated. An onyx that is white with reddish brown stripes is called a sardonyx.
Legend of Onyx
The word "onyx" means nail of a finger or claw in Greek. The Greek legend says onyx was developed due to Cupid cutting the fingernails of Venus while she was sleeping. Cupid left the clippings on the ground and the gods turned them into stone because no part of the heavenly body could die. Thus, these stones became known as onyx.
Associations
Onyx is considered the birthstone of December and in the zodiac, it is considered the birth stone of the sign of Leo. For anniversaries, onyx is traditionally given on the 7th and 10th wedding anniversary celebrations.
Uses
Onyx is often used as a black background for fine jewelry, such as cameos, and it is used to complement white marcasite jewelry. According to gemselect.com, black onyx is the most inexpensive and common of all black gemstones. More expensive black gemstones that are used in jewelry include sapphire, spinel, jet, diamond and tourmaline.