Identification guide · Minerals
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.
Onyx identification checklist
| Color | Black and white banded, Brown and white banded, Red and white banded | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Waxy to vitreous | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Translucent to opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 6.5–7 | It scratches glass and a steel knife and cannot be scratched by a steel file. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | None | |
| Fracture | Conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.6–2.65 | Ordinary rock-like heft (specific gravity about 2.625), similar to quartz. |
| Crystal system | Trigonal (cryptocrystalline) |
Extra tests for Onyx
Glass check. Onyx shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Onyx look-alikes and how to tell them apart
Both sit near Mohs 6.5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Onyx vs Chalcedony →
Both sit near Mohs 6.5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Onyx vs Agate →
Both sit near Mohs 6.5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Onyx vs Fire Agate →
Both sit near Mohs 6.5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Onyx identification FAQ
How can you tell if a rock is Onyx?
Check the color (black and white banded, brown and white banded, and red and white banded), the waxy to vitreous luster, a Mohs hardness of 6.5–7 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of 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 the difference between Onyx and Chalcedony?
Onyx (Mohs 6.5–7, waxy to vitreous luster) and Chalcedony (Mohs 6.5–7, waxy to vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Onyx vs Chalcedony comparison for a side-by-side table.
Is Onyx worth anything?
Entry-level Onyx specimens sell for around $2, typical collector pieces about $20, and premium examples $150 or more depending on size, quality and locality.
What color is Onyx?
Onyx is most often black and white banded, brown and white banded, and red and white banded. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
