igneous
Likely banded obsidian or dark volcanic glass
Obsidian, predominantly rhyolitic volcanic glass (SiO₂-rich), possibly with devitrified bands and phenocrysts
AI-generated identification · may be incorrectBlack to gray-brown, vitreous luster, conchoidal fracture, no visible cleavage, and Mohs hardness about 5–5.5; the pale layered zones may reflect flow banding or weathered/devitrified material. The image suggests a dense, glassy volcanic specimen, though exact identification is uncertain from appearance alone.
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Physical properties
Black to gray-brown, vitreous luster, conchoidal fracture, no visible cleavage, and Mohs hardness about 5–5.5; the pale layered zones may reflect flow banding or weathered/devitrified material. The image suggests a dense, glassy volcanic specimen, though exact identification is uncertain from appearance alone.
Formation & geological history
Forms when silica-rich lava cools too rapidly for crystals to develop, commonly during late volcanic eruptions. It is geologically young relative to most crystalline rocks, ranging from recent to millions of years old.
Uses & applications
Used for ornamental objects, lapidary work, tools, and historical blades; attractive banded varieties are collected and occasionally made into jewelry.
Geological facts
Obsidian can produce exceptionally sharp edges when fractured. Flow bands record movement and compositional variation within viscous rhyolitic lava.
Field identification & locations
Look for a glassy surface, sharp conchoidal edges, uniform darkness, and lack of cleavage; a streak test is generally absent or pale. Common localities include volcanic regions of the western United States, Mexico, Armenia, Japan, and Iceland.
