igneous

Obsidian

Obsidian (principally SiO2 - Silicon Dioxide)

Hardness: 5-5.5 (Mohs scale); Color: Typically jet black, but can be green, brown, or flashy (sheen/rainbow); Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.4

Hardness: 5-5
Luster: Vitreous (glassy)

Identified March 8, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of silica-rich felsic lava, which prevents the growth of mineral crystals. It is found in geologically young areas of volcanic activity, mostly from the Pliocene to the Holocene epochs.

Uses & applications

Used in jewelry (beads and cabochons), ornamental carvings, and high-end surgical scalpel blades because it can be fractured to a molecularly thin edge. Historically used for arrowheads and scrapers.

Geological facts

Obsidian is technically a natural volcanic glass rather than a mineral because it lacks a crystalline structure. Native Americans famously used obsidian from the Glass Buttes for tools and weapons.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture and sharp edges. Common locations include Oregon, California, and Iceland. Collectors look for unique varieties like 'Snowflake' or 'Rainbow' obsidian.

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

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral