Properties
| Category | Volcanic glass |
|---|---|
| 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 |
From the Wikipedia article
Obsidian is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is 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, giving them a high viscosity. The high viscosity inhibits the 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.
Source
Source record: crystal-compendium/obsidian/minerals.json. Fetched from English Wikipedia by the operator 2 October 2026.
https://en.wikipedia.org/wiki/Obsidian — revision id 1374861758. Text and properties from Wikipedia, licensed CC BY-SA 4.0.