igneous
Obsidian
Obsidian (Volcanic Glass, ~70% SiO2)
Hardness: 5-5.5 on Mohs scale; Color: typically jet black but can be grey or reddish; Luster: vitreous (glassy); Structure: amorphous (non-crystalline); Fracture: conchoidal (shell-like curves); Specific Gravity: 2.3-2.6
- Hardness
- 5-5
- Color
- typically jet black but can be grey or reddish
- 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 on Mohs scale; Color: typically jet black but can be grey or reddish; Luster: vitreous (glassy); Structure: amorphous (non-crystalline); Fracture: conchoidal (shell-like curves); Specific Gravity: 2.3-2.6
Formation & geological history
Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Usually found in active or recently active volcanic regions such as the American Pacific Northwest, Mexico, and Iceland. Most deposits are less than 20 million years old.
Uses & applications
Historically used for arrowheads and blades; currently used in high-end surgical scalpels, jewelry (cabochons), and as a decorative stone in architecture and landscaping.
Geological facts
Because it is a volcanic glass, obsidian is metastable and eventually devitrifies into quartz over millions of years. It produces edges much sharper than steel knives because its structure is molecularly smooth rather than crystalline.
Field identification & locations
Identify in the field by its distinct glass-like appearance, extremely sharp edges where broken, and conchoidal fracture pattern. Commonly found in rhyolitic volcanic flows in the Western USA, Japan, and Italy.
