igneous

Obsidian

Volcanic glass (SiO2 - Silicon Dioxide with impurities)

Hardness: 5-5.5 on Mohs scale; Color: typically black (but can be green, brown, or striped); Luster: Vitreous/Glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Hardness: 5-5
Color: typically black (but can be green, brown, or striped)
Luster: Vitreous/Glassy

Identified March 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of felsic lava, preventing the growth of mineral crystals. It is common in areas with recent volcanic activity, dating from the Cenozoic era back to much older periods, though it devitrifies over millions of years.

Uses & applications

Historically used for arrowheads and blades; currently used in jewelry (as a gemstone), ornamental sculptures, and in high-precision surgical scalpel blades due to its ability to hold an edge thinner than steel.

Geological facts

Obsidian fractures with extremely sharp edges. Unlike most rocks, it is technically a glass and a mineraloid rather than a mineral because it lacks a crystal structure. It was highly prized in pre-Columbian Mesoamerica for cutting tools and mirrors.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture patterns and glassy texture. Common locations include the United States (Oregon, California, Arizona), Iceland, Mexico, and Italy. 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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral