igneous

Obsidian

Amorphous volcanic glass (70-75% SiO2)

Hardness: 5-5.5 on Mohs scale; Color: Black, sometimes dark green or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (has Conchoidal fracture); Specific Gravity: 2.35-2.6.

Hardness: 5-5
Color: Black, sometimes dark green or brown
Luster: Vitreous/Glassy

Identified April 4, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of silica-rich (felsic) lava, which prevents crystal growth. Usually found in geologically young volcanic areas.

Uses & applications

Used in gemstone jewelry, ornamental carvings, and high-quality surgical scalpel blades because it can be fractured to a microscopic edge. Historical use for arrowheads and tools.

Geological facts

Obsidian is technically not a mineral because it is amorphous and lacks a regular crystalline structure. It is relatively unstable over geological time and eventually devitrifies into crystalline minerals.

Field identification & locations

Identify by its extremely smooth, curved (conchoidal) glass-like fracture surfaces and sharp edges. Found in volcanic regions like the American West (Oregon, California), Iceland, and Mexico.

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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