igneous

Obsidian

Obsidian (Volcanic Glass, ~70–75% SiO2)

Hardness: 5–6; Color: Typically black, can be grey, green, or brown; Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6

Hardness: 5–6
Color: Typically black, can be grey, green, or brown
Luster: Vitreous/Glassy

Identified May 10, 2026More igneous →

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Physical properties

Formation & geological history

Formed from extrusive igneous activity when felsic lava cools extremely rapidly with minimum crystal growth. Typically occurs in margins of rhyolitic lava flows or volcanic domes.

Uses & applications

Used in gemstone jewelry, ornamental carvings, and high-quality surgical scalpel blades. Historically used for making sharp tools like arrowheads and knives.

Geological facts

Because it doesn't have a crystal structure, obsidian edges can reach molecular thinness, making it sharper than high-quality steel scalpels. It is metastable and will eventually turn into quartz over millions of years.

Field identification & locations

Identify by its characteristic shell-like (conchoidal) fracture and glassy shine. Commonly found in volcanic regions like the American West (e.g., Yellowstone, Glass Butte) and Iceland.

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