Igneous Rock

Black Obsidian

Obsidian, volcanic glass (Amorphous SiO₂)

Hardness: 5-5.5 (Mohs scale); Color: Typically jet black, but can be dark brown, green, or even rarely iridescent with inclusions; Luster: Vitreous (glassy); Crystal Structure: Amorphous (lacks a defined crystal structure); Cleavage: None (It fractures conchoidally, like glass); Specific Gravity: 2.35-2.60.

Hardness: 5-5
Luster: Vitreous (glassy)

Identified September 13, 2025More igneous rock →

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Physical properties

Formation & geological history

Formed from rapidly cooling felsic lava (high silica content) that solidifies before significant crystal growth can occur. This rapid cooling often happens when lava flows into water or through an eruption in a cool environment. Geological Age: Varies depending on location, as it's typically formed relatively recently in volcanic history (Cenozoic Era to present day).

Uses & applications

Industry: Historically used for cutting tools (razors, arrowheads, spear points) due to its extremely sharp edges when fractured. Modern uses include surgical scalpels. Jewelry: Polished and faceted for use in necklaces, pendants (as seen in the image), and other ornamental items. Construction: Rarely used in construction due to its brittle nature.

Geological facts

Obsidian is essentially natural volcanic glass. Despite its common black color, some obsidian contains tiny gas bubbles or mineral inclusions that can create iridescence (rainbow obsidian) or patterns (snowflake obsidian, apache tears). It has been used by various ancient cultures for tools and weapons for millennia, celebrated for its keen edge.

Field identification & locations

Identify it in the field by its glassy texture, conchoidal fracture (smooth, curved breaks), and typically dark color. It's often found in areas of recent volcanic activity. Common locations include parts of the United States (Oregon, California, Arizona), Mexico, Iceland, Italy, Greece, Japan, and parts of Africa. Tips for collectors: Be mindful of its sharp edges if you find it in raw form. Check for varieties like snowflake or rainbow obsidian for unique specimens.

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