igneous
Obsidian
Volcanic glass
Hardness: 5 to 5.5 on the Mohs scale; Color: Black, dark brown, or occasionally green/grey; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35 to 2.60.
- Hardness
- 5 to 5
- Color
- Black, dark brown, or occasionally green/grey
- 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 to 5.5 on the Mohs scale; Color: Black, dark brown, or occasionally green/grey; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.35 to 2.60.
Formation & geological history
Obsidian forms as an extrusive igneous rock when felsic lava extruded from a volcanic vent cools rapidly with minimal crystal growth. It is typically associated with rhyolitic lava flows.
Uses & applications
Historically used for making cutting tools, arrowheads, and blades due to its extremely sharp conchoidal fracture. Today it is used in jewelry, ornamental objects, and experimentally in surgical scalpels.
Geological facts
Because it cools so rapidly without forming a crystal structure, obsidian is technically a mineraloid rather than a true mineral. It has been prized by ancient civilizations worldwide for weapon-making and ceremonial items.
Field identification & locations
Easily identified by its dark, glass-like appearance and razor-sharp conchoidal fracture edges. Commonly found in regions with recent volcanic activity, such as the western United States, Mexico, Italy, and Japan.
