gemstone

Black gemstone, likely onyx or black spinel

Possible black chalcedony (SiO₂) or spinel (MgAl₂O₄)

AI-generated identification · may be incorrect

Opaque to slightly translucent, black, and glassy to waxy; the rectangular polished stone is mounted in jewelry, so hardness and cleavage cannot be confirmed. Onyx/chalcedony has Mohs hardness 6.5–7; spinel is 7.5–8 and typically has cubic crystals with no cleavage.

Identified August 15, 2026More gemstone →

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At a glance

Common name: Black gemstone, likely onyx or black spinel
Scientific name: Possible black chalcedony (SiO₂) or spinel (MgAl₂O₄)
Category: gemstone
Identified: August 15, 2026
Method: Standard

AI Estimated Value

Approximately $10–$100 for common onyx jewelry; $100–$1,000+ if confirmed as quality natural black spinel, depending on metal, craftsmanship, and provenance.

Not an official valuation

AI estimate for general interest only — not an appraisal. Actual value depends on condition, provenance, and market demand. Verify with a professional before buying, selling, or insuring.

This identification was generated by AI and may contain errors. Verify important details before relying on them — and never use an AI result to judge whether a specimen is safe to handle. Read the full disclaimer

Physical properties

Formation & geological history

Black chalcedony forms when silica-rich groundwater fills cavities and fractures, while black spinel crystallizes in metamorphic rocks such as marbles; both are geologically ancient and occur in many ages and regions.

Uses & applications

Used as a cabochon in rings, pendants, and decorative jewelry; black onyx is also used for carvings and ornament, while spinel is valued as a durable gemstone.

Geological facts

The image shows a faceted or polished rectangular black stone in a vintage-style setting, but lighting and mounting obscure diagnostic features. Uniformly opaque black material sold as “onyx” is often dyed chalcedony or glass imitation.

Field identification & locations

Check with a loupe for natural banding, inclusions, and facet reflections; a refractive-index or hardness test by a jeweler is needed for identification. Black onyx is common in Brazil, Uruguay, India, and Madagascar; black spinel occurs in Sri Lanka, Myanmar, and Tanzania.