Igneous

Obsidian

Obsidian (Volcanic Glass, SiO2 + impurity)

Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6

Hardness: 5-5

Identified April 5, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs scale), Color: Jet black to dark grey, Luster: Vitreous/Glassy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.35-2.6

Formation & geological history

Formed through the rapid cooling of silica-rich (felsic) lava, which prevents the growth of crystals. It is typically associated with rhyolitic volcanic activity from the Cenozoic to the present, as the glass eventually devitrifies into minerals over millions of years.

Uses & applications

Used in the manufacture of extremely sharp scalpel blades, jewelry, decorative carvings, and historically for tools/weapons like arrowheads and knives.

Geological facts

Obsidians can have extremely sharp edges, thinner than the finest steel scalpel. Ancient Aztecs used obsidian for ceremonial daggers and mirrors. Some varieties show sheen like 'Rainbow' or 'Gold Sheen' obsidian due to gas bubbles or mineral inclusions.

Field identification & locations

Identify in the field by its distinct glassy appearance and shell-like (conchoidal) fractures. Found in volcanic regions like the American West (Glass Buttes), Mexico, Iceland, and Japan.

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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