igneous
Tumbled mahogany obsidian
Volcanic glass, predominantly rhyolitic silica (SiO₂), with iron-oxide inclusions
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; dark brown to black with reddish-brown, mahogany-colored patches and vitreous luster. Amorphous and noncrystalline, lacking cleavage; conchoidal fracture and specific gravity around 2.35–2.6.
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Physical properties
Hardness about 5–5.5 Mohs; dark brown to black with reddish-brown, mahogany-colored patches and vitreous luster. Amorphous and noncrystalline, lacking cleavage; conchoidal fracture and specific gravity around 2.35–2.6.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, with iron oxides producing the mahogany coloration. Most gem-quality material is Quaternary in age and commonly associated with volcanic fields in Mexico and the western United States.
Uses & applications
Used for lapidary carvings, cabochons, beads, polished stones, and inexpensive ornamental objects; rarely used structurally. Tumbled pieces generally have modest collecting value.
Geological facts
The reddish patches are iron-rich inclusions or flow-related concentrations within volcanic glass. Despite its glassy appearance, natural obsidian is a geological rock rather than manufactured glass.
Field identification & locations
Identify by its smooth glassy surface, opaque black-brown color, mahogany swirls or patches, and conchoidal fracture; it is often lighter and less crystalline than jasper. Common sources include Mexico, Oregon, Utah, and Arizona.
