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Onyx

Last updated: April 2026

True onyx is a parallel-banded chalcedony with alternating black and white layers. In trade, 'black onyx' almost exclusively refers to uniformly black dyed chalcedony or agate — banded material with red-brown and white bands is called sardonyx. Virtually all commercial black onyx is dyed; accepted with disclosure. Separate from black jet (SG ~1.3), black obsidian (RI ~1.50, conchoidal fracture), and black spinel (RI 1.718, SG 3.60, isotropic).

Onyx has a specific gravity of 2.61 and a refractive index of 1.530–1.543, with a Mohs hardness of 6.5–7.0. It is aggregate.

Physical & Optical Properties

RI Range1.530–1.543
SG Range2.58–2.64
SG Typical2.61
Hardness (Mohs)6.5–7
Crystal SystemTrigonal
Optic CharacterAggregate
Dispersion0.013
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsBlack
SpeciesQuartz
Black

Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.

Through the refractometer

Onyx varies 1.530 – 1.543 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORotate the stone through a full 360°, watching the boundary. SEEOne boundary that does not move as you rotate — and it isoften vague rather than crisp. Where it sits is not fixedfor this species: published at 1.530 – 1.543, and the stonedrawn here reads 1.536, between the dashed marks. MEANSThis is an aggregate — a mass of microscopic crystals — andon this instrument it looks exactly like a singly refractivestone. The refractometer cannot tell the two apart. Thepolariscope can: an aggregate stays light through a fullrotation, where a singly refractive stone stays dark. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

AgateAmethystAmetrineAventurineBloodstoneCarnelianChalcedonyChrysopraseCitrineHawk's EyePrasioliteRock CrystalRose QuartzSmoky QuartzTiger's EyeChrysocolla Chalcedony

Key Differentiators

Common Simulants

Treatments

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying an onyx? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Onyx is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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