igneous
Possible tumbled obsidian
Volcanic glass, primarily rhyolitic silica glass (SiO₂-rich)
AI-generated identification · may be incorrectSmall dark gray-to-black, glassy-looking, smoothly worn fragment with conchoidal-looking edges; hardness about 5–5.5 Mohs, vitreous luster, no visible crystals or cleavage, and specific gravity around 2.35–2.65.
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Physical properties
Small dark gray-to-black, glassy-looking, smoothly worn fragment with conchoidal-looking edges; hardness about 5–5.5 Mohs, vitreous luster, no visible crystals or cleavage, and specific gravity around 2.35–2.65.
Formation & geological history
Likely formed when silica-rich lava cooled extremely rapidly, preventing crystal growth; volcanic obsidian is generally Quaternary to younger, though older examples also occur. The rounded shape suggests natural abrasion or tumbling.
Uses & applications
Used for ornamental objects, jewelry, cabochons, tools, and historically extremely sharp cutting blades. Small common pieces have modest collector value.
Geological facts
Obsidian commonly breaks with curved, shell-like fractures and can resemble dark glass, but it is a natural volcanic material rather than manufactured glass.
Field identification & locations
Common localities include volcanic regions of the western United States, Mexico, Iceland, Japan, and Armenia. Confirm with a fresh fracture, glassy luster, lack of bubbles, and absence of crystal grains; image-based identification is not definitive.
