Onyx Value, Price, and Jewelry Information

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Onyx is a type of chalcedony, which is itself a form of microcrystalline quartz. Onyxes have straight, nearly parallel bands or layers of color, which allow skilled gem carvers to cut away material to create cameos and intaglios with extraordinary depth and contrast.

Arabic Onyx

Although onyxes may occur with bands of various colors, white and black layers are the most familiar. Gems with a black base color and a white upper layer are known as "true" or "Arabic onyx."

Sardonyx

Onyx with white and reddish, brownish, or yellow layers is known as sardonyx, the traditional August birthstone. Cornelian or carnelian onyx has a red base color and a white upper layer. Ancient civilizations from the Near East and Europe created many amazing sardonyx carvings.

Nicolo Onyx

Onyx with a very thin layer of white, which can appear almost blue against a black base color.

Black Onyx

Onyxes usually form with very thin layers of color. Although, very rarely, they can occur in nature with layers thick enough to cut a stone as a solid color, most solid black onyxes sold today are actually dyed black chalcedonies.

Onyx Jewelry

During the Victorian Grand Period (1861-1885), onyxes were popular gemstones, especially for mourning jewelry. Over the centuries, these stones have accumulated a somber and sinister body of folklore.

Although onyx and the color black in general still carry funereal associations in Western cultures, jewelry styles since Victorian times have utilized onyx's dark and opaque appearance to complement transparent colored gems as well as diamonds in a variety of pieces.

Contemporary jewelers have also seized on the stark appearance and vitreous polish luster of onyxes to make them center stones in their own right. Despite onyx's opacity, gem cutters will often facet this material because the flat surfaces can show a remarkable sheen.

Is Onyx a Type of Agate?

Some mineralogical reference materials consider onyxes a variety of agate, itself a variety of chalcedony. According to these sources, the term "onyx" should refer to black, brown, or white banded agates or those with monochromatic light-and-dark bands.

Alternatively, other sources (including the IGS) consider agates and onyxes both banded varieties of chalcedony, only agates have concentric or curved bands and onyxes straight or nearly parallel bands. These distinctions hold regardless of the color of the bands or layers.

Of course, in either case, onyx remains a chalcedony, whether as a variety or sub-variety.

Onyx Misnomers

The popularity of onyx, however, had led to some misuse of the term. The most common example of this is so-called "Mexican onyx," "limestone onyx," or "onyx marble." This material is actually a type of banded calcite found in limestone caves.

Calcite is a distinct gem species, not a variety of chalcedony quartz. They have different optical and physical properties. Notably, calcites have perfect cleavage and a much lower hardness than onyxes, thus making them much more fragile. Artisans have used this banded calcite material for millennia to carve decorative items. Today, consumers can still find many objects for sale labelled as "onyx marble." Although this remains a widely used term, keep in mind that this material isn't onyx at all.

Other gemstones have gained "onyx" descriptions based on their appearance, too. For example "onyx obsidian" is a banded variety of that natural glass. "Onyx opals" have alternating layers of common and precious opal. Of course, quartz (onyx), obsidian, and opal are all distinct gem species.

Identifying Characteristics

Black, opaque gemstones pose numerous challenges for identification. This 1996 GIA article offers some testing recommendations.

Synthetics

Laboratories can synthesize quartz, and this material does appear in jewelry. However, consumers looking for onyxes are more likely to encounter simulants, natural gemstones or lab-created materials that look like onyxes but with distinct chemistry and/or crystal structures. 

For example, synthetic materials such as black, opaque, devitrified glass as well as black, opaque cubic zirconia have been used as onyx substitutes. Natural gemstones such as augite and opaque, black spinel have also been offered or misidentified as onyxes. Oddly enough, spinel makes a more durable and generally more expensive jewelry stone than onyx. However, as a black gemstone, onyx has more name recognition than spinel, so that may motivate some vendors to sell black spinels as onyxes.

Even in ancient times, onyxes inspired some skillful imitations. For example, the Roman scholar Pliny the Elder described how some "have discovered how to make genuine stones of one variety into false stones of another." For example, so-called sardonyxes were assembled from layers of black, white, and vermillion stones so carefully that "the artifice cannot be detected."

Enhancements

Dyeing onyxes isn't a recent development, nor is it limited to solid black specimens. Since these gems may have very thin color layers, dyes may be used to enhance them. Chalcedony is extremely porous, so it readily absorbs dyes of any color.

To create black onyxes, manufacturers typically soak grayish slabs of chalcedony in a sugar solution, then heat them in sulfuric acid. However, this procedure may leave tiny carbon particles in the pores and creates black color only to a depth of a few millimeters. Although this is a stable process, polishing such treated material may remove the black layer.

Dyeing is so pervasive that most gem certificates will note that black onyxes are "probably dyed." Unless a report explicitly states a black onyx is free of treatments, assume it's treated.

Sources

Onyxes occur across the globe. Some notable sources include the following locations:

• Argentina; Australia; Brazil; Canada; China; Czech Republic; France; Germany; India; Indonesia; Madagascar; Russia; Scotland, United Kingdom; United States; Uruguay; Yemen.

Stone Sizes

Like other chalcedonies, onyxes can form in masses several pounds in weight and many inches in diameter.

Care

Onyxes can make durable jewelry stones. Their hardness of 7 means they can resist scratching from one of the most common hazards of everyday wear: household dust. With no cleavage and a tough tenacity, they can resist chips and physical blows well.

Gem cutters should note that silicious mineral dust from quartz, including onyxes, may cause silicosis and silicotuberculosis. Although some debate exists over whether non-crystalline silica will cause these diseases, gem cutters should still use dust masks and have proper ventilation for their workspaces. See our article on lapidary health hazards and safety tips for more information. Wearing and handling finished stones should pose no risks.

Since onyxes often receive dye treatments, clean them only with a soft brush, mild detergent, and warm water. Don't use ultrasonic or other mechanical cleaning systems. Keep in mind that vintage and antique onyxes as well as ancient jewelry pieces may have been treated, too.

Consult our gemstone jewelry cleaning guide for more care recommendations.