igneous
Tumbled black obsidian
Obsidian (volcanic glass), primarily rhyolitic silica glass
AI-generated identification · may be incorrectUsually black, smooth, and vitreous with conchoidal fracture; hardness about 5–5.5 Mohs, no cleavage, amorphous rather than crystalline, and specific gravity about 2.35–2.65. The glossy rounded surface indicates tumbling or polishing.
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Physical properties
Usually black, smooth, and vitreous with conchoidal fracture; hardness about 5–5.5 Mohs, no cleavage, amorphous rather than crystalline, and specific gravity about 2.35–2.65. The glossy rounded surface indicates tumbling or polishing.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically in volcanic domes and flows; most examples are Quaternary to recent, though obsidian occurs throughout geologic time.
Uses & applications
Used for ornamental stones, cabochons, beads, carvings, and historically extremely sharp tools and projectile points. It has modest jewelry and lapidary value.
Geological facts
Obsidian can produce edges sharper than many steel blades, but it is brittle. Black color commonly results from microscopic iron-oxide inclusions.
Field identification & locations
Identify by its glassy luster, uniform dark color, light weight relative to metal, and conchoidal fracture; natural surfaces may show flow bands or small bubbles. Common sources include Mexico, the western United States, Armenia, Italy, and Japan.
