igneous

Obsidian

Volcanic Glass (SiO2)

Hardness: 5-5.5; Color: Jet black to dark grey; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4

Hardness: 5-5
Color: Jet black to dark grey
Luster: Vitreous (glassy)

Identified January 15, 2026More igneous →

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Physical properties

Hardness: 5-5.5; Color: Jet black to dark grey; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 2.4

Formation & geological history

Formed by the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It is typically geologically young because it eventually crystallizes or hydrates into perlite.

Uses & applications

Historically used for knapping sharp tools and weapons; currently used in jewelry (gemstones), high-end surgical scalpels, and home decor.

Geological facts

Unlike most rocks, obsidian is not made of minerals but is technically a mineraloid. It can be found with white needle-like inclusions of cristobalite known as 'snowflake obsidian'.

Field identification & locations

Identify by its characteristic glassy texture and curved, razor-sharp conchoidal fractures. Commonly found in volcanic regions such as the Cascades (USA), Iceland, Mexico, and the Lipari Islands.

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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