Obsidian

Hardness: 5-6 on Mohs scale. Color: Commonly black, but can be dark brown, green, or even rarely red or blue depending on impurities. Luster: Vitreous (glassy). Crystal structure: Amorphous (lacks crystalline structure). Cleavage: None, exhibits conchoidal fracture (shell-like, curved breaks). Specific Gravity: 2.35-2.50.

Obsidian is an extrusive igneous rock formed from rapidly cooling felsic lava. The rapid cooling prevents atomic diffusion and crystallization, resulting in a glassy texture. It forms in volcanic environments where lava flows are exposed to the atmosphere or water, and is common in geologically recent volcanic activity. Most obsidian deposits are geologically young, typically Cenozoic in age.

Historically, due to its sharp edges when fractured, obsidian was extensively used for cutting tools, weapons (arrowheads, spear tips), and surgical instruments by ancient civilizations. In modern times, it is used in jewelry (polished cabochons, beads), decorative objects, and occasionally as a scalpel material in specialized surgeries because it can produce an edge many times sharper than high-quality steel surgical scalpels. It also has aesthetic appeal for collectors.

Obsidian knives were found in Neolithic sites. It was highly valued by many cultures, including the Mayans and Aztecs, who traded it extensively. Despite being glass, it is considered a mineraloid because it does not have a crystalline structure. Hydration dating, a technique measuring the water absorbed by the surface of obsidian, can be used to determine the age of archaeological artifacts made from it.

Field identification: Look for its distinct glassy luster, dark color (often black), and characteristic conchoidal (shell-like) fracture pattern. Its sharp edges are also a key indicator. It's commonly found in areas of past or present volcanic activity worldwide, such as the Western United States (e.g., Oregon, California, Yellowstone), Mexico, Peru, Iceland, and parts of the Mediterranean. For collectors, fresh, unbroken pieces with good luster and interesting colors or patterns (like snowflake obsidian, which contains white spherulites of cristobalite) are highly sought after.