igneous

Obsidian

Obsidian (principally SiO2)

Hardness: 5.0–6.0 on Mohs scale; Color: Black (most common), can be brown, green, or banded; Luster: Vitreous (glassy); Crystal structure: Amorphous (non-crystalline glass); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.4

Hardness: 5
Color: Black (most common), can be brown, green, or banded
Luster: Vitreous (glassy)

Identified April 11, 2026More igneous →

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Physical properties

Formation & geological history

Obsidian is an extrusive igneous rock formed when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is found in geologically recent volcanic areas, often in 'obsidian flows' or cooling edges of rhyolitic domes.

Uses & applications

Used in ornamental jewelry, decorative items, and historically for edged tools like arrowheads. In modern times, it is used to produce thin, sharp surgical scalpel blades.

Geological facts

Because it is a glass, it is metastable and eventually devitrifies (crystallizes) over time, which is why obsidian older than the Cretaceous period is rarely found. Some varieties like 'Snowflake Obsidian' contain inclusions of cristobalite.

Field identification & locations

Identify in the field by its distinct glassy luster and smooth, curved (conchoidal) fracture surfaces that resemble the inside of a seashell. Commonly found in volcanic regions of Oregon, California, Iceland, Mexico, and Japan.

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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