igneous
Polished black obsidian
Volcanic glass; primarily silica-rich rhyolitic composition (SiO₂)
AI-generated identification · may be incorrectUsually jet-black with vitreous luster, smooth conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; specific gravity roughly 2.35–2. hyphen 2.65. The glossy, amorphous appearance fits polished obsidian, though the image alone cannot exclude black glass or another dark stone.
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Physical properties
Usually jet-black with vitreous luster, smooth conchoidal fracture, no visible crystals, and Mohs hardness about 5–5.5; specific gravity roughly 2.35–2. hyphen 2.65. The glossy, amorphous appearance fits polished obsidian, though the image alone cannot exclude black glass or another dark stone.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, mainly during Quaternary to recent volcanic eruptions. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically extremely sharp tools and projectile points; not generally used as structural stone.
Geological facts
Obsidian is technically a natural glass rather than a fully crystalline rock. Polishing produces the mirror-like surface visible here.
Field identification & locations
Identify by its lightweight feel, glassy luster, sharp conchoidal edges, and absence of grain; a scratch test and inspection for flow bands or tiny bubbles can help distinguish it from manufactured glass. Common varieties include snowflake, mahogany, and rainbow obsidian.
