igneous
Obsidian (black volcanic glass)
Obsidian; predominantly rhyolitic volcanic glass, mainly SiO₂ with minor feldspar, quartz, and iron oxides
AI-generated identification · may be incorrectTypically black, gray, or brown with vitreous luster; hardness about 5–5.5 Mohs, no cleavage, amorphous rather than crystalline, and conchoidal fracture producing razor-sharp edges. The glossy, curved fracture surfaces and flow-like texture are consistent with obsidian.
Identified More igneous →
Explore Obsidian (black volcanic glass) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Typically black, gray, or brown with vitreous luster; hardness about 5–5.5 Mohs, no cleavage, amorphous rather than crystalline, and conchoidal fracture producing razor-sharp edges. The glossy, curved fracture surfaces and flow-like texture are consistent with obsidian.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, usually at volcanic domes and lava flows. Most specimens are Quaternary to younger, though obsidian occurs throughout geologic time.
Uses & applications
Used historically for blades, arrowheads, and surgical tools; today it is collected, carved, and occasionally used in jewelry and decorative objects.
Geological facts
Obsidian is technically a natural glass, not a fully crystalline mineral. Hydration-rim dating can sometimes estimate the age of archaeological artifacts.
Field identification & locations
Common localities include Iceland, Mexico, the western United States, Armenia, Italy, and Japan; distinguish it from slag by its natural flow banding and clean conchoidal fracture. The image may also show weathering or surface coating, so composition cannot be confirmed visually alone.
