Igneous

Obsidian

Obsidian (Volcanic Glass), primarily SiO2 (Silicon Dioxide)

Hardness: 5-5.5 on Mohs scale; Color: typically jet black, can be dark green or brown; Luster: Vitreous (glassy); Crystal Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.6.

Hardness: 5-5
Color: typically jet black, can be dark green or brown
Luster: Vitreous (glassy)

Identified March 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of felsic (silica-rich) lava, preventing the growth of mineral crystals. Generally found in geologically young volcanic areas (Cenozoic era) because it is metastable and eventually devitrifies into crystals.

Uses & applications

Used historically for sharp tools and weapons (arrowheads/scalpels); currently used in jewelry (cabochons), ornamental carvings, and high-end surgical blades.

Geological facts

Obsidian can be sharper than high-quality steel because it splits down to the molecular level. 'Snowflake obsidian' contains white inclusions of cristobalite, while 'Rainbow obsidian' shows colorful iridescence.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture, sharp edges, and glassy surface. Common in volcanic regions like the American West (Iceland, Mexico, Lipari). Collectors look for uniform deep black or unique inclusions.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

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Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral