igneous
Polished black obsidian
Volcanic glass; predominantly rhyolitic glass, approximately 70–80% SiO₂
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; opaque black, vitreous to glassy luster, conchoidal fracture, no cleavage, and typically ~2.3–2.6 specific gravity. The smooth, shaped surface appears polished or tumbled, with a natural-looking flow seam or fracture line.
Identified More igneous →
Explore Polished black obsidian in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness about 5–5.5 Mohs; opaque black, vitreous to glassy luster, conchoidal fracture, no cleavage, and typically ~2.3–2.6 specific gravity. The smooth, shaped surface appears polished or tumbled, with a natural-looking flow seam or fracture line.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, generally during Quaternary volcanic eruptions. Common source regions include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for ornamental carvings, cabochons, beads, tools, and historically extremely sharp blades; it is also a popular lapidary and metaphysical stone. Not generally used as structural construction stone.
Geological facts
Obsidian can produce exceptionally sharp edges because conchoidal fractures create thin, smooth flakes. Some varieties show rainbow, sheen, snowflake, or mahogany patterns from microscopic inclusions.
Field identification & locations
Identify by its glassy appearance, smooth curved fracture surfaces, lack of visible crystals, and inability to scratch ordinary glass reliably unless sharp-edged. A steel knife may leave little or no mark, while a quartz crystal can scratch it; confirm that it is not industrial glass by examining natural flow banding and fracture texture.
