Obsidian (likely Golden Sheen Obsidian or Rainbow Obsidian, given the subtle sheen visible on some beads)

Hardness: 5-5.5 (Mohs scale), Color: Typically opaque black, but can be dark brown, green, or rarely blue/red. Often with a subtle iridescence or sheen (e.g., golden sheen obsidian, rainbow obsidian)., Luster: Vitreous (glassy), Crystal structure: Amorphous (lacks crystalline structure), Cleavage: None (exhibits conchoidal fracture), Specific gravity: 2.3-2.6

Formation process: Volcanic glass formed from the rapid cooling of viscous lava (felsic lava, typically rhyolitic) with minimal crystal growth. This rapid quenching occurs when lava flows into water or through the eruption of lava flows into the atmosphere, preventing the formation of atomic crystalline structures. Geological age: Can be found in geological formations ranging from Tertiary to Quaternary periods, often associated with recent volcanic activity. Environments where formed: Along the margins of rhyolitic lava flows, volcanic domes, and in areas of explosive volcanic eruptions.

Industry: Historically used for cutting tools, weapons (arrowheads, spear points, knives), and surgical blades. Modernly, it has niche uses in surgical scalpels due to its extremely sharp edge. Jewelry: Widely used for beads, cabochons, carvings, and polished ornamental objects due to its attractive glassy luster and ability to be polished smoothly. Construction: Not typically used in construction due to its brittle nature and conchoidal fracture. Collecting value: Valued by collectors, especially varieties like rainbow obsidian, snowflake obsidian, and mahogany obsidian for their unique patterns and colors.

Obsidian forms such a sharp edge that it has been used for surgical scalpels in modern medicine. The incredibly sharp edges are due to its amorphous structure, which allows it to fracture on an atomic scale. Its use dates back to prehistoric times for tools and weapons due to its superior sharpness. The Aztecs famously used obsidian for their macuahuitl, a wooden sword with embedded obsidian blades.

How to identify in the field: Look for a glassy texture and conchoidal fracture (shell-like breaks with concentric ridges). It is typically dark and opaque, though thin pieces may be translucent. It has a relatively low hardness for a rock but is harder than glass. Common locations found: Found in areas with recent volcanic activity worldwide, including the United States (Oregon, California, Idaho, New Mexico, Arizona, Wyoming), Mexico, Iceland, Japan, Turkey, Kenya, and Guatemala. Tips for collectors: Search near rhyolitic volcanic flows and domes. Look for outcrops with a characteristic glassy texture and sharp edges. Be cautious, as fractured obsidian can be extremely sharp. Varieties can be identified by specific inclusions or optical effects (e.g., small, white, radially clustered crystals of cristobalite form 'snowflake obsidian'; microscopic inclusions of magnetite or other minerals cause the golden or rainbow sheen).