Minerals · common

Onyx

Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz, distinguished from agate by its straight, parallel bands rather than agate's characteristically curved layers. Most commercial onyx sold in black and white banded slabs is actually dyed to enhance contrast, since naturally occurring true onyx with strong black-and-white banding is relatively uncommon. Chemically identical to other quartz varieties, onyx owes its distinctive look entirely to microscopic banding formed during slow silica deposition.

Photo: James St. John · Wikimedia Commons · CC BY 2.0

Onyx properties at a glance

Type	Minerals
Chemical formula	SiO2
Mohs hardness	6.5–7
Crystal system	Trigonal (cryptocrystalline)
Luster	Waxy to vitreous
Streak	White
Cleavage	None
Fracture	Conchoidal
Specific gravity	2.6–2.65
Transparency	Translucent to opaque
Common colors	Black and white banded, Brown and white banded, Red and white banded
Rarity	common

Geology & background

Onyx forms when silica-rich groundwater deposits successive layers of chalcedony within cavities in volcanic or sedimentary rock, with each band recording a subtle shift in the mineral-laden solution's composition or flow rate over time. Because the layers form in flat, gravity-controlled bands rather than the wavy patterns typical of agate, onyx tends to occur in more tabular or vein-like deposits rather than rounded nodules. Historic sources include Brazil, Uruguay, Pakistan, and Mexico, with Mexican onyx, technically a banded calcite rather than true silica onyx, widely used for carved bookends and decorative objects.

Collectors and buyers should be aware that much commercial onyx jewelry is dyed chalcedony rather than naturally banded material, since natural true onyx with dramatic black and white contrast is genuinely scarce. Because onyx shares quartz's hardness and durability, it has been carved into cameos, seals, and jewelry for thousands of years, valued as much for workability as for appearance. Sardonyx, a related banded variety combining onyx with reddish-brown sard layers, has its own long history in engraved gemstone carving dating back to ancient Rome.

How to identify Onyx

Onyx shows straight, parallel color bands, typically black and white, though red, brown, and gray banded material also occurs, with a waxy to vitreous luster typical of chalcedony. Its streak is white, hardness matches quartz at about 6.5 to 7, and it lacks cleavage, breaking with a smooth conchoidal fracture. The straight, ribbon-like banding is the main clue separating it from agate, which shows curved or concentric bands instead. Much commercial onyx is artificially dyed, so uniform, unnaturally vivid black coloring can be a sign of treatment rather than natural formation.

Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
Use multiple photos when possible, including a fresh surface and a close view of texture.

Where Onyx is found

Reported from multiple localities worldwide; verify locality against specimen provenance when available.

How much is Onyx worth?

Entry-level specimen	$2
Typical collector specimen	$20
Premium specimen	$150+

Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.

Common uses

Collector specimens
Education
Field identification
Industrial or lapidary use when suitable

Interesting facts

True black-and-white banded onyx is rare enough that most commercial onyx jewelry is actually dyed gray chalcedony rather than naturally banded stone.
Ancient Romans carved sardonyx, a reddish relative of onyx, into cameo portraits that remain prized by antique collectors today.
What is commonly sold as decorative Mexican onyx is technically banded calcite, not true silica-based onyx at all.