igneous
Obsidian (likely black volcanic glass)
Obsidian; predominantly rhyolitic glass, mainly SiO₂
AI-generated identification · may be incorrectBlack, glassy to vitreous luster with conchoidal-looking fractures; hardness about 5–5.5 Mohs, no cleavage, amorphous/noncrystalline structure, and typically specific gravity 2.35–2. Corroded or rough surfaces can appear resinous and uneven.
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Physical properties
Black, glassy to vitreous luster with conchoidal-looking fractures; hardness about 5–5.5 Mohs, no cleavage, amorphous/noncrystalline structure, and typically specific gravity 2.35–2. Corroded or rough surfaces can appear resinous and uneven.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, usually during Quaternary to older volcanic eruptions. Common sources include volcanic fields in the western United States, Mexico, Iceland, Armenia, Japan, and Italy.
Uses & applications
Used historically for razor-sharp blades and arrowheads; today it is popular for carvings, cabochons, decorative objects, and mineral collections. It has little large-scale construction value.
Geological facts
Fresh fractures can be exceptionally sharp, sharper than many steel blades at microscopic scale. The pictured specimen could also be another black volcanic material, such as basalt or slag, without tests.
Field identification & locations
Identify by its uniform glassy appearance, conchoidal fracture, and lack of visible grains; a streak test is usually white or colorless. Check for vesicles, crystals, or magnetic attraction to distinguish basalt, magnetite, or industrial slag.
