igneous
Polished mahogany obsidian
Obsidian, natural volcanic glass; predominantly silica-rich rhyolitic glass (SiO₂)
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; glossy vitreous luster, dark brown to reddish-brown with lighter mahogany patches. Amorphous (no crystal structure) and lacks cleavage; breaks with sharp conchoidal surfaces. The photo’s polish and pattern are consistent with mahogany obsidian.
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Physical properties
Hardness about 5–5.5 Mohs; glossy vitreous luster, dark brown to reddish-brown with lighter mahogany patches. Amorphous (no crystal structure) and lacks cleavage; breaks with sharp conchoidal surfaces. The photo’s polish and pattern are consistent with mahogany obsidian.
Formation & geological history
Forms when silica-rich lava cools too quickly for crystals to grow, commonly in volcanic flows and dome margins. It is geologically young, generally Quaternary in notable obsidian deposits.
Uses & applications
Used for cabochons, beads, carvings, and decorative objects; historically fashioned into sharp blades and tools.
Geological facts
Its brown-red patches result from iron-oxide inclusions or dispersed iron-rich material within the volcanic glass.
Field identification & locations
Identify by its glassy sheen, smooth curved fracture marks, and brown-black mottling; it occurs near rhyolitic volcanoes, including deposits in Mexico and the western United States. A scratch test should be done cautiously because obsidian can chip and leave very sharp edges.
