igneous
Likely obsidian (black volcanic glass)
Obsidian; predominantly rhyolitic volcanic glass, mainly SiO₂
AI-generated identification · may be incorrectUsually black, brown, or gray with vitreous luster and conchoidal fracture; hardness about 5–5.5 Mohs, no cleavage, and specific gravity around 2.3–2.6. The specimen appears highly glossy and irregular, though the image cannot confirm whether it is natural obsidian or black slag/glass.
Identified More igneous →
Explore Likely obsidian (black volcanic glass) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Usually black, brown, or gray with vitreous luster and conchoidal fracture; hardness about 5–5.5 Mohs, no cleavage, and specific gravity around 2.3–2.6. The specimen appears highly glossy and irregular, though the image cannot confirm whether it is natural obsidian or black slag/glass.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary to older volcanic eruptions. Common source regions include western North America, Mexico, Iceland, Italy, Japan, and Armenia.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads; today it is used for decorative objects, cabochons, carvings, and occasionally surgical blades. Common pieces have modest collector value.
Geological facts
Obsidian is technically volcanic glass rather than a fully crystalline mineral. Snowflake, sheen, rainbow, and mahogany varieties contain distinctive inclusions or flow textures.
Field identification & locations
Identify by conchoidal, razor-sharp fracture, glassy luster, lack of visible crystals, and a consistent volcanic flow texture; it should not show bubbles typical of manufactured glass or slag. Typical value is about $5–$30 for a small common specimen, or $30–$150+ for attractive patterned material.
