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Onyx is a type of chalcedony with alternating bands of color, commonly brown, red, white, and black. The color is created through impurities, but can also be artificially treated. Onyx is used in jewelry and ornaments, and should be cared for to avoid cracking.
Onyx is a semi-precious form of chalcedony that has alternating straight bands of color, most commonly brown, red, white, and black. Chalcedony is the group name for a large family of stones, all made up of silicon dioxide, also known as quartz. To be classified as chalcedony, a stone must be milky and translucent to transparent, often layered or banded with color. The color of onyx, as with other forms of chalcedony, is created through impurities that leach into the rock as it forms.
The most classic color combination for onyx has red or brown bands alternating with white. This form is also sometimes called sardonyx, because the red bands resemble sard, another type of chalcedony. More rarely, the black bands will be mixed, or the onyx will be mostly black. Black onyx is highly prized for use in jewelry and ornaments. Other colors can also be found, although they are less common.
Unfortunately, the color of onyx is often created through chemical treatment and heat. It can be difficult to tell if a stone has been dyed or heat treated, as dealers are not required to disclose this. In the case of an onyx that has been through several hands, the dealer may not know if the stone is natural or not. Although dyed colors should not fade or distort, some people prefer the idea that their jewelry is made from natural rocks in their true colors, rather than artificial ones.
Various cuts are used to display onyx in jewelry. Usually a cabochon or other large round cut will be made to show the bands of color in the stone. A high polish is achieved through rock tumbling, and the stone is often set in a deep, strong setting which will further enhance the natural beauty of the stone. Sardonyx in particular is also used to produce cameos, because the uniform bands of color lend themselves well to classic two-tone carvings.
To care for onyx, people need to be careful not to bump or bang their jewelry, as it can cause the stone to crack. Onyx is reasonably strong, although exposure to excessive sunlight and harsh chemicals should probably be avoided. If the onyx is set on a metal, the setting should be checked periodically to make sure it is not loose. Onyx beads can benefit from periodic cleaning and polishing to remove dirt, and should be taken to a jeweler for cleaning.
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