igneous

Obsidian

Obsidian (Volcanic glass), SiO2 (70% or more)

Hardness: 5-5.5 (Mohs scale); Color: Typically black but can be green, brown, or gray; Luster: Vitreous (glassy); Crystal structure: Amorphous (none); Cleavage: None (conchoidal fracture); Specific gravity: 2.4

Hardness: 5-5
Color: Typically black but can be green, brown, or gray
Luster: Vitreous (glassy)

Identified May 30, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of silica-rich lava which prevents crystal growth. It is found in geologically young volcanic areas (Cenozoic era).

Uses & applications

Used for making surgical scalpel blades, jewelry (pendants, beads), ornamental carvings, and historically for sharp tools like arrowheads.

Geological facts

Obsidian fractures with extremely sharp edges—thinner and sharper than high-quality steel scalpels. It is not a true mineral because it lacks a crystalline structure.

Field identification & locations

Identify by its distinct glassy luster and conchoidal (shell-like) fracture patterns. Found in volcanic regions such as the American Northwest, Iceland, and Mexico.

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Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

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