Igneous
Obsidian (specifically Rainbow or Silver Sheen Obsidian)
Volcanic Glass (predominantly SiO2)
Hardness: 5–5.5 Mohs scale; Color: Black with metallic silver or rainbow iridescence; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Specific Gravity: 2.35–2.6; Conchoidal fracture.
- Hardness
- 5–5
- Color
- Black with metallic silver or rainbow iridescence
- Luster
- Vitreous (glassy)
Identified More igneous →
Explore Obsidian (specifically Rainbow or Silver Sheen 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: Black with metallic silver or rainbow iridescence; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Specific Gravity: 2.35–2.6; Conchoidal fracture.
Formation & geological history
Formed when extrusive felsic lava cools rapidly with minimal crystal growth. The 'sheen' or 'rainbow' effect is caused by microscopic inclusions of gas bubbles or minerals like magnetite or feldspar oriented along flow layers. Typically found in young volcanic regions (Cenozoic to Holocene).
Uses & applications
Used primarily in jewelry (beads, cabochons), ornamental carvings, and historically for sharp tools/scalpels due to its ability to hold an edge thinner than steel.
Geological facts
Because it is glass, obsidian is metastable and eventually 'devitrifies' into crystalline quartz; thus, it is rarely found in rocks older than the Cretaceous period. It was one of the most highly traded materials in prehistoric times for making arrowheads and mirrors.
Field identification & locations
Identify by its glass-like texture, conchoidal (shell-like) fractures at the drill holes, and the unique internal 'glow' when rotated under a strong light source. Commonly found in Iceland, Mexico, and the Western United States (Oregon/California).
