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
Related: Quartz Varieties
Key Differentiators
- Opaque black body color — most common form in trade
- True onyx has parallel black and white bands; commercial "black onyx" is almost always dyed chalcedony
- Hardness 6.5–7 — scratches glass; warm to touch (vs cold glass/ceramic)
- SG 2.58–2.64 (distinguishes from black glass ~2.5 and black spinel ~3.60)
- Acetone test: color transfer indicates dyed material (nearly universal for commercial black onyx)
Common Simulants
- Black spinel: RI 1.718 (vs ~1.535); SG 3.60 (vs 2.61); isotropic; vitreous luster.
- Black glass (jet glass): Isotropic; SG ~2.4–2.5; gas bubbles possible; conchoidal fracture.
- Jet: Organic material; SG 1.10–1.35 (much lower than onyx ~2.62); dramatically lighter weight — heft test is diagnostic; warm to touch; smooth, polished surface.
- Black Tourmaline (Schorl): Strongly pleochroic; RI 1.624–1.644; SG 3.06; vitreous luster; often shows striations.
Treatments
- Dyed black (nearly universal for commercial black onyx — widely accepted with disclosure)
Price Context
Price context is approximate. GemID is not an appraisal tool. Results are indicators, not certified valuations.
Measurement Guides
Identifying an onyx? GemID walks through these tests in order — RI, SG, fluorescence, and more.
Try GemID Free →Studying for the GIA, FGA, or FEEG practical? Onyx is one of the 130 species in GemID’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.
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