igneous
Obsidian
Obsidian (Volcanic Glass)
Hardness: 5-6 (Mohs scale), Color: Black/Grey, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
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Physical properties
Hardness: 5-6 (Mohs scale), Color: Black/Grey, Luster: Vitreous (glassy), Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6
Formation & geological history
Obsidian forms when silica-rich felsic lava cools so rapidly that crystals do not have time to grow. It is typically found in volcanic regions from the Tertiary to Quaternary geological periods.
Uses & applications
Historically used for arrowheads and blades; currently used in high-end surgical scalpels, jewelry (cabochons), and ornamental home decor.
Geological facts
Because it is not a mineral (it lacks a crystal structure), it is technically a 'mineraloid'. It is often sharper than high-quality steel scalpels because it breaks down to a nearly molecular edge.
Field identification & locations
Identify by its characteristic shiny, glassy surface and sharp, curved (conchoidal) edges. Found commonly in the Western USA, Iceland, Mexico, and Italy.
