igneous

Obsidian

Obsidian (Volcanic Glass, SiO2)

Hardness: 5-5.5 on Mohs scale; Color: typically black (but can be brown, green, or banded); Luster: vitreous (glassy); Crystal Structure: amorphous (non-crystalline); Cleavage: none, exhibits conchoidal fracture; Specific Gravity: 2.35-2.6.

Hardness: 5-5
Color: typically black (but can be brown, green, or banded)
Luster: vitreous (glassy)

Identified April 1, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of felsic (silica-rich) lava, which prevents the growth of mineral crystals. It is most commonly found in volcanic areas of recent geological age (Tertiary and Quaternary).

Uses & applications

Historically used for making sharp tools like arrowheads and knives; currently used as a gemstone for jewelry, in scalpel blades for delicate surgery, and for ornamental or decorative purposes.

Geological facts

Because it is a glass and lacks a stable crystalline structure, obsidian is metastable and eventually devitrifies into microscopic mineral crystals over millions of years. This is why very old (Precambrian) obsidian is rarely found.

Field identification & locations

Identify in the field by its intense glassy luster and curved, shell-like (conchoidal) fracture surfaces that produce extremely sharp edges. Commonly found in Oregon, California, and Mexico.

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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