What is Onyx?

Onyx is a banded variety of the oxide mineral chalcedony. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved chaotic bands and onyx has parallel bands.

Its multi-layered structure is similar to that of agates. When the stone has a black base and a white upper layer, it's called onyx. When the base is brown, it is called sardonyx while carnelian-onyx has a red base.

Onyx comes through Latin (of the same spelling), from the Ancient Greek ὄνυξ, meaning "claw" or "fingernail". Onyx with flesh-colored and white bands can sometimes resemble a fingernail.

Natural Onxy. Photo credit: UCL Geology Collections

A typical onyx consists of two or more black and white strata, whilst the term sardonyx is applied to the stone if it contains red or brown bands.

Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black. Black onyx is perhaps the most famous variety, but is not as common as onyx with colored bands.

Onyx isn’t an actual family of gemstones, rather a term used to describe layered stones that contain bands of black and white. The key difference here is that sardonyx is technically an onyx stone that’s layered with sard bands. In other words, sard is a variety of onyx, but it has a brown base instead of black.

Onxy agate with Quartz from Uruguay, at the Mineralogy Gallery in Paris.
Photo: Camille Gévaudan

Artificial treatments have been used since ancient times to produce both the black color in "black onyx" and the reds and yellows in sardonyx. Most "black onyx" on the market is artificially colored.

Onyx is the traditional birthstone for February. It is associated with relaxation and comfort.

Onyx is popular as a carving stone, since clever use of alternating bands of black and white can create a highly appealing carving.

Onyx is a gemstone found in various regions of the world including Greece, Yemen, Uruguay, Argentina, Australia, Brazil, Canada, China, Czech Republic, Germany, Pakistan, India, Indonesia, Madagascar, Latin America, the UK, and various states in the US.

Onyx is a variety or type of: Agate, which is a variety of Chalcedony, which is a variety of Quartz.

Onyx Agate from Brazil.
Photo: James St. John

Properties of Onyx Agate:

Crystal system: Trigonal
Color: Various
Cleavage: no cleavage
Fracture: Uneven, conchoidal
Mohs scale hardness: 6.5–7
Luster: Vitreous, silky
Streak: White
Specific gravity: 2.55–2.70