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Onyx is a variety of chalcedony and has a hexagonal/trigonal and microcrystalline inner structure. It has a refractive index of 1.544-1.553 and comes in multiple color varieties from banded white to black. It has a waxy (not glassy) luster and becomes vitreous after polishing. Onyx scores a seven on the hardness index and is extremely wearable, which makes it a good choice for a first semiprecious stone.

This stone also presents a conchoidal fracture in its unpolished state and does not have any cleavages. Onyx is not heated sensitive and will not fracture, melt, or easily deform in the presence of intense heat unless the heat reaches volcanic levels, which is unusual if a person is simply wearing the onyx as jewelry.

Commercial onyx can be enhanced through controlled heating and the application of dyeing or staining with what is considered as ‘safe substances’ like sugar solutions and sulfur. True onyx is not transparent and presents uniaxial optics.

The name is derived from the Greek word that means “claw,” a clear reference to its appearance when struck by light.

As a subtype of chalcedony, onyx presents a complex internal color structure that usually figures in a parallel weave of different color bands. Skill craftsmen can carve and cut onyxes to make specific color bands more visible, though it takes a lot of skill to do so.

Four main types of onyxes 

In order to be truly familiar with genuine onyxes, it would be best to go through the color varieties one by one.

  1. Black onyxes with white layer – This type of onyx is called “Arabic onyx” and is the most widely recognized and popular type of onyx. It is also called a “true onyx.”
  1. White, red, brown and yellow color bands – Such an onyx would be categorized as sardonyx and has a base that is usually reddish. The top layer of sardonyx tends to be white. Sardonyx has been around for a long time and ancient carvings exist in Europe featuring this subtype.
  1. Nicolo – The Nicolo onyx features an almost pure black onyx backdrop with a very thin white layer that may appear blue under regular light.
  1. Black – As onyxes are naturally banded with multiple colors, it is very rare for a stone to be cut into a singular, solid color with no additional color bands. Onyx is considered “black onyx” if the color bands are quite thin and almost not visible unless the structure is examined very, very closely.

Onyxes and agates

In some books, onyxes are also considered a variety of agate. However, its main family, if you will, remains chalcedony. Onyxes are classically termed as those which have parallel lines that run the gamut from black to brown to white.

Onyx is differentiated from agates because agates have color bands that have a tendency to curve, instead of maintaining a linear track.

In some references, it is said that instead of being a sub-variety of agate, onyx and agate are classified as both being part of the chalcedony group. Either way, onyx will remain a sub-variety of chalcedony, whichever reference is being used.

Frequently asked questions: onyx edition

Is there such a thing as Mexican onyx or marble onyx?

No, there isn’t. However, the misnaming of objects and similar-looking minerals has been done for such a long time that it has become a common (albeit bad) practice. What is being called Mexican onyx isn’t onyx at all, but instead it is a subtype of calcite that is mined because of its attractive color bands.

While banded, it is not chalcedony, and therefore, it can never be genuine onyx. Again, while the banded calcite can be ornamental and beautiful in its own right, onyx and calcite are quite distinct from each other.

So basically when someone offers you “onyx marble” or something to that effect, it’s not really onyx, but likely another gem species being masqueraded as the more popular and more valuable onyx.

Can you synthesize onyx in a laboratory setting?

Yes. Scientists have long discovered how to synthesize onyx substitutes. These onyx lookalikes are aptly called “simulants” and they can look exactly like onyxes, but their inner structures that are only visible with a microscope would be different from mined onyx.

One good example of an onyx simulant would be devitrified glass, which has a deep, solid color and is opaque, as would a true black onyx.

Cubic zirconia has also been tapped to create simulants that look the part, as black onyxes are quite rare and expensive. Be careful when buying onyx beads and other onyx jewelry as you may be offered other natural minerals that kind of look like onyx, but really aren’t because they are from a different gem species.

Black spinel, for instance, is commonly offered as “black onyx.” Ironically though, black spinel has a higher durability rating than genuine onyx, so it’s not really a bad onyx substitute at all. However, the jeweler has to be straightforward – if the jewelry is a stimulant or a lookalike; they have to be upfront about it.

Are black onyxes dyed? 

This is another point of confusion for many onyx buyers because black onyx is rare, but there are plenty of black onyxes on the market. What manufacturers actually do to supply enough of the semiprecious stone to the market is they dye it. If you can remember, onyx is notable for its multiple color bands.

When this semiprecious stone is harvest, it is possible to carve to specific color layers and apply the dye there. The coloring process involves the application of sugar solution and allowing the sugar solution to react with sulfur. However, the process doesn’t change the onyx from the surface all the way to its core.

So what usually happens is you will only have a few millimeters of black onyx, then the color will begin to change again, reflecting the older color configuration. May retailers will say that their black onyx has likely been dyed because the procedure is so common in the industry.