igneous
Black tumbled stone, likely obsidian
Volcanic glass; chiefly amorphous SiO₂
AI-generated identification · may be incorrectSmall, smoothly rounded, opaque black specimen with glossy vitreous luster. Obsidian has no crystal structure or cleavage, hardness about 5–5.5 Mohs, and specific gravity about 2.35–2.65; the image alone cannot exclude black onyx, basalt, or glass.
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Physical properties
Small, smoothly rounded, opaque black specimen with glossy vitreous luster. Obsidian has no crystal structure or cleavage, hardness about 5–5.5 Mohs, and specific gravity about 2.35–2.65; the image alone cannot exclude black onyx, basalt, or glass.
Formation & geological history
Obsidian forms when silica-rich lava cools so rapidly that crystals cannot grow, generally in volcanic settings younger than a few million years. This polished/tumbled shape is human processed.
Uses & applications
Used for ornaments, cabochons, beads, carvings, and historically exceptionally sharp tools and projectile points. Common tumbled pieces have modest collecting value.
Geological facts
Fresh obsidian breaks with a distinctive conchoidal fracture and razor-sharp edges; it may show brown, gray, rainbow, or snowflake patterns. It is not technically a mineral because it is glassy and noncrystalline.
Field identification & locations
Likely identifiable by its uniform glassy black appearance, light weight relative to metallic minerals, and conchoidal fracture if tested—but avoid breaking it. Around 50.8°N, 11.9°E, natural obsidian is not especially typical; a purchased tumbled stone or black glass is more likely than locally collected obsidian.
