Crystal & mineral guide
Onyx
SiO₂ (chalcedony)
Parallel-banded chalcedony — black onyx is a classic men's jewelry stone.
Identify Onyx with RockScan Get on the App StoreAbout 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.
