igneous
Black obsidian
Volcanic glass; predominantly rhyolitic silica-rich melt (approximately 70–80% SiO₂), lacking crystalline structure
AI-generated identification · may be incorrectUsually black with vitreous luster, conchoidal fracture, no cleavage, and hardness about 5–5.5 Mohs; specific gravity approximately 2.35–2.60. The specimen appears naturally fractured or roughly knapped, with reflective glassy surfaces.
Identified More igneous →
Explore Black obsidian in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Usually black with vitreous luster, conchoidal fracture, no cleavage, and hardness about 5–5.5 Mohs; specific gravity approximately 2.35–2.60. The specimen appears naturally fractured or roughly knapped, with reflective glassy surfaces.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, typically during Quaternary or younger volcanic eruptions. Common sources include volcanic regions of Iceland, Mexico, the western United States, Armenia, and Japan.
Uses & applications
Used historically for exceptionally sharp tools and arrowheads; today it is used for carvings, ornamental objects, jewelry, cabochons, and collector specimens.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Its sharp conchoidal edges can be keener than surgical steel, though brittle.
Field identification & locations
Identify it by its uniform glassy black appearance, sharp curved fracture, and lack of visible grains or crystals; it should not show vesicles like most volcanic rocks. Avoid confusing it with industrial glass or black slag, which may contain bubbles and flow textures.
