Obsidian (likely mahogany or black obsidian)

Hardness about 5–5.5 Mohs; black to brown with possible mahogany mottling, vitreous luster, no visible crystals or cleavage, conchoidal fracture, and typical specific gravity near 2.3–2.6.

Rapid cooling of silica-rich lava prevented crystal growth, generally in volcanic domes and lava flows during the Cenozoic or younger periods. The specimen appears naturally fractured or roughly shaped, with a glossy glassy surface.

Used historically for exceptionally sharp cutting tools and arrowheads; today it is collected, carved, polished, and occasionally used in jewelry and ornamental objects.

Obsidian edges can be sharper than surgical steel, though they are brittle and prone to chipping. Brown-and-black varieties are commonly called mahogany obsidian.

Identify by its glassy luster, lack of crystalline grains, light weight, and curved conchoidal breaks; it commonly occurs near rhyolitic volcanic fields in Mexico, the western United States, Iceland, and Armenia. A scratch test and examination of a fresh fracture help distinguish it from dark crystalline rocks.