igneous

Obsidian

Volcanic Glass (predominantly SiO2)

Hardness: 5–5.5 (Mohs scale); Color: Black (jet black); Luster: Vitreous/Glassy; Crystal Structure: Amorphous (lacks crystal structure); Cleavage: Conchoidal (shell-like) fracture; Specific Gravity: 2.4

Hardness: 5–5
Color: Black (jet black)
Luster: Vitreous/Glassy

Identified April 13, 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. It is typically geologically young (Cenozoic) because it eventually devitrifies into crystalline rock over millions of years.

Uses & applications

Historically used for arrowheads and blades. Currently used in jewelry, decorative items, and high-precision surgical scalpel blades because it can be fractured to an edge much thinner than steel.

Geological facts

Because it is an amorphous solid (glass) rather than a crystalline structure, it is often called a 'mineraloid' rather than a true mineral. Large flows can be found at the Obsidian Cliff in Yellowstone National Park.

Field identification & locations

Identify by its characteristic conchoidal fracture (curved, glass-like breaks) and extremely sharp edges. Commonly found in volcanic regions such as the American Northwest, Mexico, and Iceland. Collectors look for unique varieties like snowflake or mahogany obsidian.

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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