Crystal & mineral guide
Onyx
SiO₂ (chalcedony)
Parallel-banded chalcedony — black onyx is a classic men's jewelry stone.
Identify Onyx with RockScanAbout Onyx
True onyx shows straight parallel banding (vs agate's curved banding). Most commercial 'black onyx' is dyed chalcedony/agate — acceptable in trade when disclosed. Popular for signet rings and inlays across Gulf and Western markets. Cameos traditionally used white-on-black layered onyx.
Onyx belongs in a crystal identifier workflow because photo cues — color zoning, crystal habit, and surface luster — combine with measurable traits like Mohs 6.5–7 and specific gravity 2.58–2.64. Use RockScan’s camera to get a first-pass ID, then confirm with the checklist below.
Onyx properties
| Mohs hardness | 6.5–7 |
|---|---|
| Crystal system | Trigonal (microcrystalline) |
| Luster | Vitreous to waxy |
| Color | Black, or black-and-white banding |
| Composition | Parallel-banded chalcedony; black often dyed |
| Formation | Silica banding in cavities; much black onyx is treated agate |
| Occurrence | Brazil, India, Uruguay, Mexico |
| Specific gravity | 2.58–2.64 |
| Cleavage | None |
How to identify Onyx
- Straight parallel bands when banded
- Uniform jet black may indicate dye
- Hardness 7; cool dense feel vs plastic
Collector notes
- Sardonyx combines reddish and white layers
- Cameo carving historically relied on layered onyx
Value context: Natural banded pieces and fine cameo blanks price above dyed black slabs.
Onyx FAQ
How do you identify Onyx?
Onyx (SiO₂ (chalcedony)) is typically recognized by black, or black-and-white banding color, vitreous to waxy luster, Mohs hardness 6.5–7, and trigonal (microcrystalline) crystal system. RockScan can suggest Onyx from a photo and surface these checks on the result card.
What is Onyx made of?
Onyx composition: Parallel-banded chalcedony; black often dyed. It forms via silica banding in cavities; much black onyx is treated agate and is found in Brazil, India, Uruguay, Mexico.
Is Onyx valuable?
Natural banded pieces and fine cameo blanks price above dyed black slabs.
