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.
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.
Full Onyx identification guide: checklist, home tests and look-alikes →
Onyx vs similar stones
Not sure it isn't something else? Side-by-side comparisons with the stones most often confused with Onyx.
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.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
Onyx FAQ
What is 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.
How much is Onyx worth?
Entry-level Onyx specimens sell for around $2, a typical collector specimen for about $20, and premium pieces for $150 or more. Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
How hard is Onyx?
Onyx has a Mohs hardness of 6.5–7. It scratches glass and a steel knife and cannot be scratched by a steel file.
What is Onyx used for?
Onyx is used for collector specimens, education, field identification, and industrial or lapidary use when suitable.
