igneous
Polished blue obsidian with a pale mineral crust
Obsidian (volcanic glass), likely coated or encrusted with quartz/calcite
AI-generated identification · may be incorrectObsidian is typically black to blue-black, vitreous, noncrystalline, with conchoidal fracture and Mohs hardness about 5–5.5. The pale surface appears granular and cracked, possibly a natural carbonate/silica crust or an applied coating; precise identification requires testing.
Identified More igneous →
Explore Polished blue obsidian with a pale mineral crust in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Obsidian is typically black to blue-black, vitreous, noncrystalline, with conchoidal fracture and Mohs hardness about 5–5.5. The pale surface appears granular and cracked, possibly a natural carbonate/silica crust or an applied coating; precise identification requires testing.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, mainly during the Cenozoic and younger volcanic activity. The pale material may have formed later from hydrothermal fluids or weathering.
Uses & applications
Used for ornamental objects, cabochons, carvings, and historically sharp blades; the specimen is mainly suitable for collecting. Value depends strongly on whether the pale surface is natural or artificial.
Geological facts
Blue coloration can result from microscopic inclusions, nanocrystals, or surface treatment; genuine blue obsidian is uncommon and often confused with dyed glass or coated stone.
Field identification & locations
Look for sharp conchoidal edges, glassy luster, and lack of visible crystals; a steel scratch test and magnification help, while a gemological test can distinguish glass or coating. Common sources include Mexico, the western United States, Armenia, and Turkey.
