igneous
Obsidian (black volcanic glass), possibly with brecciated or weathered fragments
Obsidian; predominantly rhyolitic volcanic glass, typically rich in SiO₂
AI-generated identification · may be incorrectUsually black to dark gray with vitreous luster, conchoidal fracture, and sharp edges; hardness about 5–5.5 Mohs, no cleavage, and glassy/amorphous structure. The metallic-looking surfaces appear to be fresh fracture faces or reflective alteration, not necessarily metallic mineralization.
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Physical properties
Usually black to dark gray with vitreous luster, conchoidal fracture, and sharp edges; hardness about 5–5.5 Mohs, no cleavage, and glassy/amorphous structure. The metallic-looking surfaces appear to be fresh fracture faces or reflective alteration, not necessarily metallic mineralization.
Formation & geological history
Forms when silica-rich lava cools rapidly, preventing crystal growth, generally during Quaternary to Cenozoic volcanic eruptions. Common sources include volcanic fields in Mexico, the western United States, Armenia, Turkey, and Japan.
Uses & applications
Used historically for exceptionally sharp blades and arrowheads; today it is a carving, lapidary, and collector material, with limited decorative use. It is not generally used as aggregate because it can be brittle and chemically reactive in concrete.
Geological facts
The pieces show classic conchoidal fracture and razor-sharp flake edges characteristic of obsidian. Some may instead be industrial glass or slag if they have bubbles, uniform coloration, or melted textures.
Field identification & locations
Identify by testing for curved fracture, glassy luster, lack of visible crystals, and sharp edges; inspect for vesicles to distinguish volcanic glass from slag. Avoid handling edges directly. Value is commonly $5–$30 for small rough fragments, or $20–$100+ for attractive, provenance-bearing specimens.
