Onyx Identification Guide

What Onyx Looks Like

True onyx is a variety of agate — a banded chalcedony (microcrystalline quartz) — in which the bands are straight and parallel rather than curved/concentric. The classic form has alternating black and white layers, but onyx can be brown-and-white (sardonyx), gray, or other tones. Much commercial "black onyx" is solid black chalcedony, often dyed to a uniform jet color. It is opaque to slightly translucent with a waxy-to-vitreous polish.

• Color: black-and-white banded; also brown/white (sardonyx), gray, solid black
• Luster: waxy to vitreous when polished
• Transparency: translucent (thin edges) to opaque
• Pattern: straight, flat, parallel bands

Step-by-Step Field ID Checklist

1. Look at band geometry: straight, parallel banding = onyx; curved/concentric banding = ordinary agate.
2. Test hardness: it scratches glass and resists a knife (Mohs ~6.5–7) — this is the key separator from carbonate "onyx."
3. Check translucency: backlight a thin edge; chalcedony onyx transmits some light.
4. Examine the break: conchoidal fracture, no cleavage.
5. Watch for dye: very even, flawless black with color pooled in tiny cracks suggests dyed chalcedony.
6. Acid check: no fizz (silica).

Key Diagnostic Tests

• Hardness: ~6.5–7; scratches glass — distinguishes true (silica) onyx from soft onyx marble (~3).
• Acid: no reaction with dilute HCl (carbonate onyx marble fizzes — the single most decisive test).
• Streak: white.
• Cleavage/fracture: none; conchoidal.
• Density: ~2.6 g/cm³.

Common Look-Alikes and How to Tell Them Apart

• Onyx marble / "Mexican onyx": the biggest confusion. That material is banded calcite/aragonite — soft (Mohs ~3, scratched by a knife) and fizzes in acid. True onyx is hard quartz and acid-inert. Always do the hardness and acid tests.
• Agate: same mineral (chalcedony) but with curved/concentric banding; onyx has straight parallel bands.
• Black obsidian: glassy, amorphous, conchoidal, softer (~5–5.5), translucent brown at edges, and not banded; onyx is harder banded chalcedony.
• Jet / black glass: jet is light and warm with a brown streak; black glass shows bubbles. Both are softer than onyx.
• Dyed agate sold as onyx: legitimate but treated; the dye is the giveaway, though it is still chalcedony (hard, acid-inert).

Where Onyx Is Found

Banded chalcedony onyx forms in cavities and gas pockets of volcanic rocks where silica-rich fluids deposit layered chalcedony, as well as in some sedimentary settings. Sources of agate/onyx include Brazil, Uruguay, India, Madagascar, the USA, and Mexico (which also produces the soft carbonate onyx marble). Much black onyx on the market is gray chalcedony (often from Brazil/India) dyed black. In the field, look for layered chalcedony fills within agate-bearing volcanic terrains.