igneous
Polished dark stone, possibly obsidian
Volcanic glass; likely rhyolitic obsidian (SiO₂-rich, noncrystalline)
AI-generated identification · may be incorrectVery dark gray-black with a glossy vitreous luster and smooth, rounded polishing; obsidian has Mohs hardness about 5–5.5, conchoidal fracture, no cleavage, and specific gravity around 2.3–2.6. The image is too dark to confirm translucency, banding, or internal texture.
Identified More igneous →
Explore Polished dark stone, possibly obsidian in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Very dark gray-black with a glossy vitreous luster and smooth, rounded polishing; obsidian has Mohs hardness about 5–5.5, conchoidal fracture, no cleavage, and specific gravity around 2.3–2.6. The image is too dark to confirm translucency, banding, or internal texture.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, typically in volcanic flows and domes from the Cenozoic to recent geologic eras. It may also be another polished dark volcanic rock, such as basalt, which requires testing to distinguish.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically sharp tools; common polished pieces generally have modest value.
Geological facts
A glassy surface and sharp conchoidal edges favor obsidian, while tiny visible crystals and a duller surface favor basalt. A streak, hardness, translucency, and close inspection of fracture are useful tests.
Field identification & locations
Common sources include Mexico, the western United States, Armenia, Turkey, and Japan. Estimated value: $5–$25 for a small polished stone, or $25–$100+ for attractive patterned or rare obsidian.
