Onyx
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).
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.
About Onyx
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).
Identifying a onyx? GemID walks through these tests in order — RI, SG, fluorescence, and more.
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