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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.

Through the polariscope

Onyx No reading ANY POSITION — THE STONE BLOCKS THE LIGHT BETWEEN CROSSED POLARS Onyx is opaque, so no light reaches the analyser. The field stays dark in every position — not because the stone is singly refractive, but because the test cannot be run on it at all. SR, DR and aggregate cannot be separated this way. Computed from optic character and transparency in gems.json. A polariscope separates singly refractive, doubly refractive and aggregate — it does not show the optic sign.

Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.

Specific gravity, and what it rules out

Onyx SG 2.58–2.64 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 30 other species in the table overlap this range, so a reading here narrows the field to 31 rather than naming the stone. This is the crowded part of the scale — the same careful measurement is worth far more higher up. Computed from the specific-gravity range in gems.json. Hydrostatic weighing — the bench method this site recommends; heavy liquids are toxic and regulated. Faint ticks are the other species in the table.

Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.

Through the dichroscope

Onyx No reading NO LIGHT THROUGH THE STONE Onyx is opaque. A dichroscope reads light that has passed THROUGH the stone, so there is nothing to split and no reading to take — the same limit that rules out the polariscope. Computed from pleochroism and pleochroic colours in gems.json. Window tints are derived mechanically from the colour words, not chosen; a neutral pair depicts no colour at all.

Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.

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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