igneous
Polished mahogany obsidian
Volcanic glass, chiefly rhyolitic in composition (SiO₂-rich amorphous silicate)
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; reddish-brown to black with a vitreous, highly reflective polish and irregular dark inclusions or flow bands. It lacks true crystal structure and has conchoidal fracture; cleavage is absent and specific gravity is approximately 2.3–2.6.
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Physical properties
Hardness about 5–5.5 Mohs; reddish-brown to black with a vitreous, highly reflective polish and irregular dark inclusions or flow bands. It lacks true crystal structure and has conchoidal fracture; cleavage is absent and specific gravity is approximately 2.3–2.6.
Formation & geological history
Rapid cooling of silica-rich volcanic lava prevented crystallization, while iron-oxide-rich zones produced the mahogany-red color. Most specimens formed during relatively young Quaternary volcanic activity, especially in the western United States.
Uses & applications
Used for lapidary carvings, cabochons, polished display pieces, ornaments, and occasionally sharp experimental tools. Common polished specimens generally have modest collector value.
Geological facts
The red coloration typically reflects hematite or other iron-oxide inclusions in black obsidian. Its glassy fracture can produce exceptionally sharp edges.
Field identification & locations
Identify it by its glassy luster, conchoidal fracture, lack of visible crystals, low-to-moderate weight, and red-black flow pattern; it should not fizz in acid. Common sources include Oregon, California, Utah, Arizona, Mexico, and Armenia.
