igneous
Polished black gemstone, likely obsidian
Volcanic glass; primarily silica-rich rhyolitic composition (approximately SiO₂)
AI-generated identification · may be incorrectTypically black, vitreous/glassy luster, conchoidal fracture, no visible crystals, Mohs hardness about 5–5.5, and specific gravity around 2.35–2.70. The smooth cabochon is human-polished; strong reflections and possible internal sheen are consistent with obsidian, though black onyx or glass are alternatives.
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Physical properties
Typically black, vitreous/glassy luster, conchoidal fracture, no visible crystals, Mohs hardness about 5–5.5, and specific gravity around 2.35–2.70. The smooth cabochon is human-polished; strong reflections and possible internal sheen are consistent with obsidian, though black onyx or glass are alternatives.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystallization, mainly during relatively young volcanic eruptions from the Cenozoic to present. Common sources include Mexico, the western United States, Armenia, Turkey, and Iceland.
Uses & applications
Used for cabochons, beads, carvings, ornaments, and historically sharp tools and weapons; less commonly used as architectural stone.
Geological facts
Obsidian is technically natural volcanic glass rather than a crystalline mineral. Rainbow, sheen, snowflake, and mahogany varieties display distinctive internal patterns or inclusions.
Field identification & locations
Identify by its glassy luster, conchoidal fracture, uniform black color, and lack of grain; a glass scratch test and microscopic examination can help distinguish it from manufactured glass or onyx. Exact identification requires testing or provenance.
