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Onyx is a type of chalcedony, or quartz with a different structure.  Its colors are black, black and white, and grey and white.  It is not particularly rare nor expensive.  In ancient times, it was much more precious than it is now.

The most well known type of onyx is black onyx - the type most often found in jewelry.  However, onyx also comes in banded grey colors and sometimes in patterns.

The most valuable onyx:

Most of the value in a peice of onyx jewelry is in the design and not in the gemstone material.  This is different from a ruby, for example, where the jewelry is designed around the expensive, precious stone.  In onyx jewelry, the emphasis is on the design and the jeweler can use onyx as a feature.  Therefore, most often, the value of a peice of onyx jewelry is more dependent on the quality of the design than on the onyx stone.

However, that being said, larger peices of onyx can add more value to jewelry than tiny onyx gemstones. 

The most important thing to look for is the quality of the surface: the polish should be shiny and reflective in an evenly manner throughout the stone's surface.  Additionally, there should not be any surface cracks or pits.  Sometimes, it is easiest to observe the gemstone's surface with a loupe (gemological magnifying glass).

The best onyx is also slightly transparent.

Onyx Treatments:

Onyx is very commonly dyed to give it a "blacker" and more even color.  Onyx, like all chalcedony, is porus and obsorbs color quite easily.  It can be difficult to distinguish onyx gemstones that have been dyed from natural gemstones, but that's okay because dyeing is considered very normal in the industry.

Clarification:

Sometimes we see stones on the market with names like "green onyx" or "red onyx."  If these are natural stones, then they are probably just other varieties of chalcedony: onyx only comes in shades of grey, black and white.

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