igneous
Polished black obsidian
Volcanic glass; primarily silica-rich rhyolitic glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectTypically jet-black, opaque, and highly vitreous with conchoidal fracture; Mohs hardness 5–5.5, no cleavage, and specific gravity about 2.3–2.6. The smoothly polished surface and glassy reflections fit obsidian, though black glass or onyx are alternatives.
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Physical properties
Typically jet-black, opaque, and highly vitreous with conchoidal fracture; Mohs hardness 5–5.5, no cleavage, and specific gravity about 2.3–2.6. The smoothly polished surface and glassy reflections fit obsidian, though black glass or onyx are alternatives.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly during Quaternary volcanic eruptions. Important sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for cabochons, beads, carvings, ornaments, and occasionally historically for exceptionally sharp blades and arrowheads; not generally used as structural stone.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Some varieties display snowflake, rainbow, sheen, or mahogany patterns from microscopic inclusions.
Field identification & locations
Identify by its glassy luster, uniform dark color, smooth conchoidal fracture, and lack of visible grains; a scratch test should not be relied on for a finished cabochon. Common polished pieces are sold worldwide from volcanic regions.
