Onyx is a semi-precious form of chalcedony which 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 of which form from silicon dioxide, also known as quartz. In order to be classed as chalcedony, a stone must be milky, and translucent to transparent, often with layers or bands of color. The color of onyx, as with other forms of chalcedony, is created through impurities which seep into the rock as it forms.
The most classic color combination for onyx has red or brown bands alternating with white ones. This form is sometimes also called sardonyx, because the red bands resemble sard, another type of chalcedony. More rarely, black bands will be mixed in, or the onyx will be primarily 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 whether or not a stone has been dyed or heat treated, as dealers are not required to disclose this. In the case of an onyx which has passed through multiple hands, the dealer may not know whether 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 artificially enhanced ones.
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A number of cuts are used to showcase onyx in jewelry. Usually a cabochon or other large, rounded cut will be made to show off 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 even bands of color lend themselves well to the classic two-toned carvings.
To care for onyx, people should take care not to bump or rap their jewelry, as it can cause the stone to crack. Onyx is reasonably hardy, although exposure to excessive sunlight and harsh chemicals should probably be avoided. If onyx is set in a metal, the setting should be checked periodically to make sure that it is not loose. Onyx beads may benefit from periodic cleaning and polishing to remove grime, and should be taken to a jeweler for cleaning.