igneous

Obsidian

Volcanic Glass (Primary component: SiO2)

Hardness: 5-5.5 (Mohs scale); Color: Typically black but can be green, brown, or mottled; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.4

Hardness: 5-5
Color: Typically black but can be green, brown, or mottled
Luster: Vitreous/Glassy

Identified March 17, 2026More igneous →

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Physical properties

Formation & geological history

Formed from extrusive igneous activity when silica-rich lava cools too rapidly for crystals to grow. Most common in the Cenozoic era due to its tendency to devitrify over geological time.

Uses & applications

Historically used for edged tools (scalpels, arrowheads). Modern uses include jewelry (cabochons), ornamental masks, and specialized surgical blades due to its molecular sharpness.

Geological facts

Obsidian fractures with an edge thinner than the sharpest steel scalpel. 'Snowflake Obsidian' forms when crystals of cristobalite begin to grow within the glass, creating white flower-like patterns.

Field identification & locations

Identify by its distinctive conchoidal (shell-like) fracture patterns and sharp edges. Found primarily in high-silica volcanic regions. Key locations include the Western United States (Oregon, California), Iceland, and Mexico.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

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.

Igneous