igneous
Obsidian
Obsidian (Volcanic Glass, SiO2 + oxide impurities)
Hardness: 5-5.5 (Mohs scale); Color: Black to dark brown (translucent when thin); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
- Hardness
- 5-5
- Color
- Black to dark brown (translucent when thin)
- Luster
- Vitreous/Glassy
Identified More igneous →
Explore 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 to dark brown (translucent when thin); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6
Formation & geological history
Formed from rapid cooling of felsic/rhyolitic lava, which prevents crystal growth. This specimen appears andesitic or rhyolitic in composition and was likely shaped via knapping.
Uses & applications
Used historically for sharp tools and weapons (arrowheads, knives); currently used in surgical scalpel blades, jewelry, and ornamental decor.
Geological facts
Because it has no crystal structure, obsidian can be fractured down to an edge nearly a single molecule thick, making it sharper than high-quality steel scalpels used in medicine.
Field identification & locations
Identify in the field by its distinctive glassy texture and conchoidal (shell-like) fracture patterns. Found in areas of recent volcanic activity such as Iceland, Mexico, and the Western USA.
