Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved bands and onyx has parallel bands.

The colors of its bands range from black to almost every color.

Commonly, specimens of onyx contain bands of black andor white.

Onyx, as a descriptive term, has also been applied to parallel banded varieties of alabaster, marble, obsidian and opal, and misleadingly to materials with contorted banding, such as Cave Onyx and Mexican Onyx.

Onyx with flesh-colored and white bands can sometimes resemble a fingernail.

Its bands are parallel to one another, as opposed to the more chaotic banding that often occurs in agates.

Black onyx is perhaps the most famous variety, but is not as common as onyx with colored bands.

Artificial treatments have been used since ancient times to produce both the black color in black onyx and the reds and yellows in sardonyx.

This material is much softer than true onyx, and much more readily available.

The majority of carved items sold as onyx today are this carbonate material.

The first-century naturalist Pliny the Elder described these techniques being used in Roman times.

Treatments for producing black and other colours include soaking or boiling chalcedony in sugar solutions, then treating with sulfuric or hydrochloric acid to carbonise sugars which had been absorbed into the top layers of the stone.

These techniques are still used, as well as other dyeing treatments, and most so-called black onyx sold is artificially treated.

In addition to dye treatments, heating and treatment with nitric acid have been used to lighten or eliminate undesirable colours.

It has also been used for intaglio and hardstone cameo engraved gems, where the bands make the image contrast with the ground.

Some onyx is natural but much of the material in commerce is produced by the staining of agate.

The German sculptor Ferdinand Preiss used Brazilian green onyx for the base on the majority of his chryselephantine sculptures.

Green onyx was also used for trays and pin dishes produced mainly in Austria often with small bronze animals or figures attached.

In Renaissance Europe, wearing sardonyx was believed to bestow eloquence.

A traditional Persian belief is that it helped with epilepsy.

Sardonyx was traditionally used by English midwives to ease childbirth by laying it between the breasts of the mother.

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ISBN 3-540-40882-7.

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Princeton, New Jersey: Van Nostrand.

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ISBN 0-7506-3173-2.

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Santa Monica, California: Gemological Institute of America.

ISBN 0-87311-012-9.

What Rome was Built with: A Description of the Stones Employed.

Rome: H. Frowde. p.

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