Obsidian
Obsidian (/əbˈsɪdi.ən, ɒb-/) is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock.
| Obsidian | |
|---|---|
| General | |
| Category | Volcanic glass |
| Identification | |
| Color | usually black; sometimes green or brown; rarely yellow, orange, red or blue |
| Fracture | Conchoidal |
| Mohs scale hardness | 5–6 |
| Luster | Vitreous |
| Specific gravity | c. 2.4 |
| Optical properties | Translucent |
| Other characteristics | Texture: Smooth; glassy |
| References | |
Obsidian is produced from felsic lava, rich in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. These flows have a high content of silica, granting them a high viscosity. The high viscosity inhibits diffusion of atoms through the lava, which inhibits the first step (nucleation) in the formation of mineral crystals. Together with rapid cooling, this results in a natural glass forming from the lava.
Obsidian is hard, brittle, and amorphous; it therefore fractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.