Obsidian

Hardness: 5-5.5 (Mohs scale); Color: Typically dark, often black or dark green, but can be brown, red, or even clear. Sheen or iridescence may be present; Luster: Vitreous (glassy); Crystal structure: Amorphous (no crystalline structure); Cleavage: None (conchoidal fracture); Specific Gravity: 2.35-2.60

Obsidian forms from the rapid cooling of felsic lava (lava rich in silica and low in iron and magnesium) without crystal growth. This typically occurs when lava extrudes from a volcano and cools very quickly, often in contact with water or air. It is geologically young, predominantly found in areas of recent volcanic activity, typically Cenozoic in age.

Historically used for tools, weapons (arrowheads, knives), and mirrors due to its sharp edges and glassy surface. In modern times, it's used for surgical scalpels (producing an edge many times sharper than conventional steel scalpels), jewelry, ornamental objects, and as a collector's specimen.

Obsidian is often called 'volcanic glass' because it lacks a crystalline structure. Despite its dark appearance, it is usually felsic in composition, similar to granite. Varieties include 'snowflake obsidian' (with white radial clusters of cristobalite), 'rainbow obsidian' (exhibiting an iridescent sheen due to microscopic inclusions), and 'golden sheen obsidian.' The sharp edges of obsidian tools were so effective that some early civilizations based their entire tool-making industries on it.

In the field, obsidian is identified by its glass-like texture, conchoidal fracture (where it breaks with characteristic curved surfaces), and generally dark color. It's found wherever recent volcanic activity has occurred, particularly in regions around the 'Ring of Fire,' such as the Western United States (e.g., Oregon, California, Arizona), Iceland, Italy, Mexico, and Japan. Collectors often look for specimens with interesting sheens or patterns.