igneous
Tumbled brown-and-black stone, likely mahogany obsidian
Volcanic glass; predominantly rhyolitic obsidian, SiO₂-rich
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; vitreous luster, opaque brown to black coloration, conchoidal fracture, and no visible crystal structure or cleavage. The brown areas may reflect iron-oxide coloration or flow banding; exact identification requires testing.
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Physical properties
Hardness about 5–5.5 Mohs; vitreous luster, opaque brown to black coloration, conchoidal fracture, and no visible crystal structure or cleavage. The brown areas may reflect iron-oxide coloration or flow banding; exact identification requires testing.
Formation & geological history
Rapid cooling of silica-rich volcanic lava prevented crystal growth, commonly during Quaternary volcanic eruptions. Mahogany obsidian is especially associated with volcanic regions of Mexico and the western United States.
Uses & applications
Used as a tumbled decorative stone, cabochon, carving material, and collector specimen; historically fashioned into sharp tools and arrowheads. Common polished pieces generally have modest commercial value.
Geological facts
Obsidian is technically volcanic glass rather than a fully crystalline mineral. Its conchoidal fracture can produce exceptionally sharp edges, sharper than many manufactured blades.
Field identification & locations
Identify by its glassy luster, smooth polish, lack of grains, and curved fracture surfaces; a steel scratch test should usually leave little or no mark. Similar brown-black jasper or slag can be distinguished by crystal texture, bubbles, or streak; common tumbled specimens occur worldwide.
