igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide)
Hardness: 5 to 5.5 on the Mohs scale. Color: Typically jet-black, but can be brown, tan, green, or translucent. Luster: Vitreous (glassy). Crystal structure: Amorphous (lacks a true crystal lattice). Cleavage: Conchoidal fracture, extremely sharp edges. Specific gravity: 2.35 to 2.60.
- Hardness
- 5 to 5
- 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: Typically jet-black, but can be brown, tan, green, or translucent. Luster: Vitreous (glassy). Crystal structure: Amorphous (lacks a true crystal lattice). Cleavage: Conchoidal fracture, extremely sharp edges. Specific gravity: 2.35 to 2.60.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It typically originates from rhyolitic lava flows.
Uses & applications
Historically used for making cutting tools, arrowheads, and spear points due to its sharp conchoidal fracture. Modern uses include surgical scalpel blades, decorative objects, and jewelry.
Geological facts
Because obsidian cools so quickly that crystals do not have time to form, it is technically a mineraloid rather than a true mineral. In ancient times, its reflective surfaces were polished to create mirrors.
Field identification & locations
Easily identified by its dark, shiny appearance, vitreous luster, and sharp conchoidal fracture edges. Found worldwide in areas with past volcanic activity, such as the western United States, Italy, Iceland, and Mexico.
