gemstone

Black rectangular gemstone pendant, likely obsidian or black glass imitation

Most plausibly black obsidian (volcanic glass; SiO₂-rich), though the image cannot exclude synthetic glass or black onyx

AI-generated identification · may be incorrect

Opaque black, vitreous luster, faceted rectangular cabochon with beveled edges; obsidian has Mohs hardness about 5–5.5, conchoidal fracture, no cleavage, and specific gravity about 2.3–2.4. The polished setting and limited view prevent definitive identification.

Identified August 18, 2026More gemstone →

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Physical properties

Formation & geological history

Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, mainly in volcanic regions of relatively young Cenozoic age. If onyx, it would instead be banded microcrystalline quartz deposited in cavities.

Uses & applications

Used for pendants, beads, ornaments, and historically sharp tools; black glass imitations are common and inexpensive. Gold-colored metal and cord indicate a finished jewelry item rather than a raw specimen.

Geological facts

Obsidian commonly shows flow bands, tiny gas bubbles, or brown/rainbow sheen, while black onyx is typically denser and may show subtle banding. A gemological refractive-index or specific-gravity test is more reliable than appearance alone.

Field identification & locations

Common sources include Mexico, the western United States, Armenia, Turkey, and Japan. Estimated value is about $5–$30 if ordinary obsidian or glass, $20–$100 if verified quality black onyx, and higher only for precious-metal mounting or provenance.