igneous
Tumbled black stone, likely obsidian
Obsidian (natural volcanic glass; mainly silica-rich rhyolitic composition)
AI-generated identification · may be incorrectBlack, vitreous to resinous luster with conchoidal-looking fractures; amorphous rather than crystalline, hardness about 5–5.5 Mohs, no cleavage, and specific gravity about 2.35–2.65. The rounded shape indicates tumbling or polishing; surface marks and reflections make precise identification uncertain.
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Physical properties
Black, vitreous to resinous luster with conchoidal-looking fractures; amorphous rather than crystalline, hardness about 5–5.5 Mohs, no cleavage, and specific gravity about 2.35–2.65. The rounded shape indicates tumbling or polishing; surface marks and reflections make precise identification uncertain.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, commonly in volcanic domes and lava flows; usually Quaternary to much older, depending on locality.
Uses & applications
Used for ornamental objects, cabochons, beads, and historically sharp tools and blades; polished pieces are common in collections.
Geological facts
Obsidian can produce exceptionally sharp edges and may show sheen, rainbow, snowflake, or mahogany patterns from inclusions and flow textures.
Field identification & locations
Field clues include very smooth glassy surfaces, curved fracture patterns, lack of visible grains, and edges that can be razor-sharp. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan; a glassy appearance alone cannot exclude slag or manufactured glass.
