igneous
Obsidian (black volcanic glass)
Obsidian; predominantly rhyolitic volcanic glass, mainly SiO₂ with minor feldspar, quartz, and iron oxides
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; opaque black, vitreous to pitchy luster, noncrystalline, with excellent conchoidal fracture and very sharp edges. The glossy, irregular surface may be natural or freshly broken.
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Physical properties
Hardness about 5–5.5 Mohs; opaque black, vitreous to pitchy luster, noncrystalline, with excellent conchoidal fracture and very sharp edges. The glossy, irregular surface may be natural or freshly broken.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically at volcanic domes and lava flows. Most specimens are Quaternary to Cenozoic, though obsidian occurs in volcanic terrains of many ages.
Uses & applications
Used for blades, arrowheads, cutting tools, ornamental objects, and cabochons; prized by collectors and occasionally used in jewelry. It is generally unsuitable for structural construction.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks a crystal lattice. Black varieties may contain microscopic gas bubbles or flow bands, and sharp flakes were important prehistoric tools.
Field identification & locations
Identify it by its glassy black appearance, conchoidal fracture, sharp margins, and lack of visible grains; it is commonly found near volcanic regions such as Mexico, the western United States, Japan, Iceland, and the Mediterranean. A streak test is usually difficult because edges are sharp.
