igneous
Polished black obsidian (probable)
Volcanic glass; predominantly rhyolitic silica glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectUsually vitreous and jet-black with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, nonmagnetic, and specific gravity around 2.35–2.65. The glossy surface and irregular conchoidal edges are consistent with polished obsidian, though the dark lighting limits certainty.
Identified More igneous →
Explore Polished black obsidian (probable) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Usually vitreous and jet-black with conchoidal fracture and no visible crystals; Mohs hardness about 5–5.5, nonmagnetic, and specific gravity around 2.35–2.65. The glossy surface and irregular conchoidal edges are consistent with polished obsidian, though the dark lighting limits certainty.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, primarily during Quaternary to older volcanic eruptions. Common sources include Mexico, the western United States, Armenia, Turkey, and Iceland.
Uses & applications
Used for ornamental carvings, cabochons, beads, arrowheads, and decorative objects; historically fashioned into exceptionally sharp blades. Common polished pieces have modest collector value.
Geological facts
Obsidian is technically natural volcanic glass rather than a crystalline mineral. Thin edges can be extremely sharp, and flow bands or small snowflake-like cristobalite inclusions may occur.
Field identification & locations
Identify by glassy luster, black color, lightweight feel, and shell-like fracture; it scratches glass but is itself scratched by harder materials. A knife-edge or conchoidal fracture helps distinguish it from black crystalline rocks such as basalt.
