Most people think of onyx as a black gemstone, but this is actually misleading in two ways. In the gemological world, the term onyx refers to layered stones that exhibit different colors in multiple layers. Usually this material is cut from agate or other types of chalcedony with even, parallel layers, usually with a lighter layer above a darker one.
Onyx may be a multilayered black and white stone, usually with a black base and a white upper layer. But it may also be brown and white, as in sardonyx; or red and white as in cornelian onyx. These types of onyx were traditionally used in gem carving and engraving, where the multiple layers in the stone could be used to produce interesting effects. Well known types of engravings include intaglio, where a negative image is created; and cameo, which is a raised relief.
What then is the black gemstone commonly known today as onyx? Usually it is agate or chalcedony which has been dyed to make it a uniform black color. It is usually cut as cabochons or beads, though more recently it has been faceted as well.
The name onyx has sometimes been used - quite incorrectly - to refer to other banded lapidary materials, such as calcite found in Mexico and Pakistan. This material is much softer than true onyx, and much more readily available. The majority of carved items sold as 'onyx' today are actually this carbonate material.
Natural onyx is similar to other cryptocrystalline quartz, with a hardness of 6.5 to 7 on the Mohs scale. Like all quartz, it is silicon dioxide with a density of 2.58-2.65 and a refractive index of 1.530-1.540.
Since black gemstones can make elegant jewelry, black onyx has become very popular. There are actually relatively few gemstones that occur in black. Onyx is the most common and least expensive of all black gemstones. Other gem varieties that occur in black include diamond, sapphire, spinel, jet and tourmaline. In the finer gems, black tourmaline is the most widely available and still sells at very affordable prices.
- First Published: February-13-2009
- Last Updated: August-22-2017
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