igneous
Black Obsidian
Volcanic Glass (SiO2 + MgO / Fe3O4)
Hardness: 5-5.5 (Mohs scale), Color: Jet black, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (Conchoidal fracture), Specific Gravity: 2.4
- Hardness
- 5-5
Identified More igneous →
Explore Black Obsidian in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5-5.5 (Mohs scale), Color: Jet black, Luster: Vitreous/Glassy, Structure: Amorphous (non-crystalline), Cleavage: None (Conchoidal fracture), Specific Gravity: 2.4
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystal structures. It is primarily found in areas of recent volcanic activity, often dating from the Cenozoic era.
Uses & applications
Used historically for cutting tools and weapons (arrowheads/scalpels); currently used in jewelry, ornamental carvings, and metaphysical collecting.
Geological facts
Obsidian is technically not a mineral because it lacks a crystalline structure; it is considered a mineraloid. It can be sharper than high-quality surgical steel scalpels due to its molecular smoothness.
Field identification & locations
Identify by its glass-like appearance and distinctive curved, shell-like (conchoidal) fractures. Found in volcanic regions such as the American West, Mexico, and Iceland. Tumbled specimens are smooth and highly reflective.
