Black Onyx Identification Guide
Identify black onyx, a uniform black chalcedony (usually dyed), by hardness, conchoidal fracture, dye clues, and how it differs from agate, obsidian, and glass.
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What Black Onyx Looks Like
Onyx is a banded variety of chalcedony with straight, parallel layers; black onyx is the solid-black form. In practice, most commercial black onyx is gray chalcedony that has been dyed solid black using a traditional sugar-and-sulfuric-acid process — an accepted, stable treatment.
- Color: uniform deep black
- Luster: waxy to vitreous; takes a high glassy polish
- Transparency: translucent on very thin edges to opaque
- Habit: massive; cut from banded chalcedony nodules
Step-by-Step Field-ID Checklist
- Check uniformity. True 'onyx' implies straight banding, but black onyx is usually evenly colored throughout.
- Hardness test. It scratches glass and steel (Mohs 6.5–7).
- Inspect for dye. Under magnification, look for color concentrated along fractures or surface pits — a sign of dyeing.
- Backlight a thin edge. Often shows a faint brown-gray translucency.
- Check fracture. Conchoidal, glassy break confirms chalcedony.
Key Diagnostic Tests
- Mohs hardness: 6.5–7; scratches glass.
- Streak: white.
- Cleavage/fracture: no cleavage; conchoidal fracture.
- Specific gravity: ~2.58–2.64.
- No acid reaction; not magnetic.
- Acetone swab: quick, surface-level dyes (rare on quality onyx) may show on a swab, though the traditional sugar-acid dye is permanent and will not transfer.
Common Look-Alikes and How to Tell Them Apart
- Black agate: same chalcedony family; agate proper shows curved/concentric banding, onyx shows straight bands or solid color. The two terms overlap heavily, and dyed black agate and black onyx can be identical material.
- Obsidian: volcanic glass, softer (Mohs ~5), lighter (SG ~2.4), brighter glassy luster, often with flow lines; onyx is harder and denser.
- Black glass: may show bubbles and mold seams; softer than onyx and will be scratched by a quartz point (onyx will not).
- Jet: organic, very soft (2.5–4), light, warm to the touch, brown streak.
- Black tourmaline: striated prismatic crystals, not a polished uniform mass.
- Plastic/resin imitations: warm, very light, may show molding marks; cannot scratch glass.
The key separators are hardness 7 (rules out obsidian, glass, jet, plastic) plus a conchoidal fracture and white streak (confirms chalcedony). Distinguishing onyx from agate is essentially about banding pattern and is partly a trade convention.
Where Black Onyx Is Found
The chalcedony used for black onyx comes mainly from Brazil, Uruguay, India, and Madagascar, then is cut and dyed (commonly in Germany's Idar-Oberstein tradition and in India). Natural solid-black chalcedony is uncommon, so most jewelry-grade black onyx is treated.
Quick Confirmation
A uniform black stone that scratches glass, breaks with a conchoidal fracture, gives a white streak, and shows a glassy polish is black onyx (dyed or natural chalcedony) — not obsidian, glass, or jet.
Frequently asked questions
How can you tell if black onyx is real?
Genuine black onyx is chalcedony: it scratches glass (hardness 6.5–7), has a white streak, breaks with a conchoidal fracture, and takes a hard glassy polish. Imitations like glass, jet, or plastic are softer and will be scratched by a quartz point; plastic also feels warm and very light.
Is black onyx dyed?
Usually yes. Most commercial black onyx is gray chalcedony dyed solid black with a traditional sugar-and-acid process that is permanent and stable. Natural solid-black chalcedony exists but is uncommon, so dyeing is the industry norm and is widely accepted.
What is the difference between black onyx and black agate?
Both are black chalcedony. Strictly, onyx has straight parallel banding and agate has curved or concentric banding, but black onyx is usually solid black and the terms overlap heavily in the trade — dyed black agate and black onyx can be the same material.
Black onyx vs obsidian — how do I tell them apart?
Onyx is crystalline quartz: harder (Mohs 7), denser, with a waxy-to-glassy polish. Obsidian is volcanic glass: softer (~5), lighter, with a very bright glassy luster and often flow lines. A quartz point scratches obsidian but not onyx.
Black Onyx identified by the community
Recent Black Onyx specimens identified with Rock Identifier.
