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.
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.
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.
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.
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.
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.
This vessel in the shape of a monkey was carved from “onyx marble,” known as tecali to the Mixtec Meso-Americans. It also sports pyrite eyes and shell teeth. 19.1 x 12.7 x14 cm, Mexico, 10th-12th century CE. The Michael C. Rockefeller Memorial Collection, Bequest of Nelson A. Rockefeller, 1979. Metropolitan Museum of Art, New York. Public Domain.
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.
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.
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.”
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.
Carving cameos in onyxes to just the right depth to reach desired colors is very challenging, especially when the stones have very thin color layers. In this sardonyx cameo from the 18th century, the carver cut through the red layer on some of the lion’s tail and rear leg. Although not the case in this piece, dyes could be used to match the color of exposed areas to the desired layer. 17.1 x 25.5 mm, Italian. The Milton Weil Collection, 1939. Metropolitan Museum of Art, New York. Public Domain. (Cropped to show detail).
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.
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.
Like other chalcedonies, onyxes can form in masses several pounds in weight and many inches in diameter.
Onyxes can make durable jewelry stones. Their hardness of 7 means they can resist scratchingfrom 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.