igneous
Black Obsidian
Volcanic Glass (felsic volcanic glass, rich in SiO2)
Hardness: 5 to 5.5 on Mohs scale. Color: Deep black with a glossy finish and gray rough cortex. Luster: Vitreous (glassy). Structure: Amorphous (non-crystalline glass). Cleavage: None (features classic sharp conchoidal fracture). Specific Gravity: ~2.4
- Hardness
- 5 to 5
- Color
- Deep black with a glossy finish and gray rough cortex
- Luster
- Vitreous (glassy)
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Physical properties
Hardness: 5 to 5.5 on Mohs scale. Color: Deep black with a glossy finish and gray rough cortex. Luster: Vitreous (glassy). Structure: Amorphous (non-crystalline glass). Cleavage: None (features classic sharp conchoidal fracture). Specific Gravity: ~2.4
Formation & geological history
Formed when felsic lava, rich in silica, cools extremely rapidly with minimal crystal growth. Typically associated with geologically recent volcanic activity.
Uses & applications
Historically used for knapping arrowheads, blades, and tools. Modernly used in surgical scalpels, lapidary art, gemstones, jewelry, and ornamental pieces.
Geological facts
Obsidian produces an edge much sharper than high-quality surgical steel blades because its structure lacks crystalline boundaries. Ancient civilizations traded obsidian across thousands of miles.
Field identification & locations
Identified by its jet-black glassy sheen, smooth curved (conchoidal) fractures, and extremely sharp edges. Commonly found in volcanic regions like Oregon, California, Iceland, Mexico, and Italy.
