Onyx (black onyx)

Onyx (black onyx) is a solid-black variety of chalcedony, a microcrystalline form of quartz with the chemical formula SiO₂. Its defining traits are full opacity, a glassy-to-waxy polish, and an even jet-black color without the parallel banding seen in other agates. Most black onyx on the market is cut as a smooth cabochon, which shows off the stone's mirror-like surface and deep color saturation.

Origin and Key Properties of Onyx (black onyx)

Chalcedony deposits that yield onyx are found in Brazil, Uruguay, India (particularly the Deccan Traps), Madagascar, Mexico, and the western United States. Brazilian and Indian material supplies most of the calibrated cabochons used in commercial silver jewelry today. Historically, Roman and Greek carvers sourced onyx from Arabia and the Indian subcontinent for signet rings, intaglios, and cameos.

Onyx measures 6.5 to 7 on the Mohs hardness scale, with a specific gravity around 2.60 and a refractive index of 1.53–1.54. It is opaque in the black variety and takes an excellent polish. While "onyx" historically described any banded chalcedony, in the jewelry trade the term now refers primarily to the solid black stone, with sardonyx (red-brown and white bands) treated as a separate variety.

Treatment disclosure matters here: most commercial black onyx is dyed. The base material is typically a pale gray chalcedony soaked in a sugar solution and then treated with sulfuric acid, which carbonizes the sugar inside the pore structure and produces a stable, permanent black color. This process has been standard practice since the 19th century and is accepted across the trade. Natural, untreated solid-black onyx exists but is uncommon in calibrated jewelry sizes.

Onyx in Sterling Silver Jewelry

The cabochon is the dominant cut for black onyx because the stone's opacity rewards a smooth domed surface rather than facets. You will also find faceted rounds, ovals, and briolettes used in rings and earrings where light play on the polished facet edges adds contrast. Inlay and flat-backed cuts appear in men's rings and signet styles. Sterling silver (.925) pairs well with onyx because the cool white metal sharpens the contrast of the black stone, and the neutral palette suits both men's and women's designs. Our onyx (black onyx) jewelry collection typically ranges from around $25 for small stud earrings to $150+ for larger statement rings and pendants.

Because onyx is dyed and relatively hard but not tough, it rewards secure bezel settings over prong settings in everyday wear pieces. Marcasite accents, hand-stamped borders, and Bali-style oxidized silverwork are common pairings that suit the stone's dark, graphic look.

Care and Cleaning

Clean onyx with warm water, mild soap, and a soft cloth. Avoid ultrasonic cleaners, steam cleaners, and household chemicals such as bleach or ammonia, which can damage the dye treatment and the stone's polish. Keep onyx out of prolonged direct sunlight and away from sudden temperature changes. Store pieces separately in a soft pouch so harder stones do not scratch the polished surface.

Frequently Asked Questions