Obsidian
Rock
Also called Volcanic glass
About it
Natural volcanic glass with a bright lustre and shell-shaped fracture that gives razor-sharp edges, used for tools since prehistory; thin edges let light through. Found at Glass Buttes, Oregon, Obsidian Cliff in Yellowstone, and in Mexico.
- Type
- Rock (igneous)
- Group
- Igneous (volcanic glass)
- Hardness (Mohs)
- 5–5.5
- Specific gravity
- 2.3–2.6
- Lustre
- vitreous (glassy)
- Colours
- black, brown, grey, green, red-brown, banded
- Transparency
- translucent, opaque
- Cleavage and fracture
- none; conchoidal fracture
Typical values: real specimens vary. For more, see mindat.org.