igneous

Obsidian

Obsidian (principally SiO2 with MgO and Fe3O4)

Hardness: 5–5.5 (Mohs); Color: Black (sometimes brown or sheen depending on inclusions); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35–2.6.

Hardness: 5–5
Color: Black (sometimes brown or sheen depending on inclusions)
Luster: Vitreous/Glassy

Identified January 11, 2026More igneous →

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Physical properties

Formation & geological history

Formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It occurs in geologically recent volcanic areas, typically from the Tertiary to Quaternary periods, as it eventually de-vitrifies over millions of years.

Uses & applications

Used historically for sharp tools and weapons (arrowheads, knives); currently used in high-end surgical scalpels, jewelry (beads and cabochons), and as a decorative stone in architecture and landscaping.

Geological facts

Obsidian is technically a 'mineraloid' rather than a mineral because it lacks a crystalline structure. Some varieties like 'Snowflake Obsidian' contain white crystal inclusions called spherulites. It has been traded across continents for over 10,000 years.

Field identification & locations

Identify by its distinct jet-black glossy surface and sharp, curved (conchoidal) edges where broken. Commonly found in volcanic regions like the American Northwest (Oregon), Iceland, Mexico, and Japan. If you hold a thin flake to the light, it is often translucent at the edges.

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral