igneous
Obsidian (likely black volcanic glass)
Obsidian; primarily amorphous volcanic silica glass, approximately SiO₂-rich rhyolitic composition
AI-generated identification · may be incorrectBlack, highly glossy vitreous luster with conchoidal fracture and razor-sharp edges; hardness about 5–5.5 Mohs, no cleavage, and noncrystalline structure. The smooth surface may be naturally fractured or polished/processed.
Identified More igneous →
Explore Obsidian (likely black volcanic glass) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Black, highly glossy vitreous luster with conchoidal fracture and razor-sharp edges; hardness about 5–5.5 Mohs, no cleavage, and noncrystalline structure. The smooth surface may be naturally fractured or polished/processed.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically in volcanic domes and lava flows. Most specimens are Quaternary to Cenozoic, though obsidian occurs in many volcanic ages.
Uses & applications
Used historically for cutting tools and arrowheads; today it is a collector material, carving stone, and occasional ornamental or jewelry material. Its sharp fracture makes it unsuitable for ordinary construction aggregate.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Black varieties may contain tiny crystals or inclusions that produce sheen, banding, or “snowflake” patterns.
Field identification & locations
Identify it by glassy reflectivity, lightweight feel, lack of visible grains, and curved shell-like fracture; it should scratch glass but not quartz. Common localities include Mexico, the western United States, Armenia, Turkey, and Japan; the image alone cannot confirm it versus dark man-made glass.
