igneous
Polished mahogany obsidian
Volcanic glass; primarily silica-rich rhyolitic composition (approximately 70–75% SiO₂)
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; black to reddish-brown mottled color, vitreous luster, amorphous (non-crystalline) structure, conchoidal fracture, and no cleavage. The specimen appears tumbled and polished, with possible iron-oxide coloration or flow banding.
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Physical properties
Hardness about 5–5.5 Mohs; black to reddish-brown mottled color, vitreous luster, amorphous (non-crystalline) structure, conchoidal fracture, and no cleavage. The specimen appears tumbled and polished, with possible iron-oxide coloration or flow banding.
Formation & geological history
Forms when silica-rich volcanic lava cools so rapidly that crystals cannot grow, commonly during Pliocene-to-Quaternary volcanic activity. Mahogany coloration results from iron-oxide inclusions or microscopic mineral-rich bands.
Uses & applications
Used for cabochons, beads, carvings, decorative objects, and lapidary collections; historically, obsidian was fashioned into exceptionally sharp blades and arrowheads.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks an ordered crystal lattice. Its curved conchoidal fractures can produce edges sharper than surgical steel.
Field identification & locations
Identify by its glassy luster, smooth polish, rounded conchoidal fractures, and black-brown mahogany pattern; common sources include Mexico, the western United States, Armenia, and Turkey. Confirm with a scratch test cautiously, since glass and some dark gemstones can look similar.