igneous

Obsidian

Volcanic glass (high silica content, usually >70% SiO2)

Hardness: 5–5.5 Mohs scale; Color: typically jet black but can be deep brown or green; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.35–2.6.

Hardness: 5–5
Color: typically jet black but can be deep brown or green
Luster: Vitreous (glassy)

Identified March 16, 2026More igneous →

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Physical properties

Formation & geological history

Obsidian forms when felsic lava extruded from a volcano cools so rapidly that individual crystals do not have time to grow. It is found in geologically young areas with volcanic activity, typically from the Tertiary or Quaternary periods.

Uses & applications

Historically used for arrowheads and blades; currently used for surgical scalpel blades (sharper than steel), ornamental jewelry, and as a semi-precious gemstone.

Geological facts

Because it is a glass, obsidian is meta-stable and over millions of years will eventually 'devitrify' and turn into crystals, meaning there is no 'ancient' obsidian from the Paleozoic era. It breaks with an edge thinner than a surgical steel blade, down to nearly molecular thickness.

Field identification & locations

Identify by its extremely smooth, curved (conchoidal) fracture surfaces and shiny black reflection. Common in Oregon, California, Iceland, Mexico, and Japan. Collectors should look for 'snowflake' patterns or 'rainbow' sheen caused by inclusions.

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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

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Magnetite

Magnetite (Fe3O4)

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