Obsidian

Hardness: 5-6 on Mohs scale. Color: Commonly black, but can be dark brown, green, or rarely blue, red, orange, or rainbow-sheen. Luster: Vitreous (glassy). Crystal Structure: Amorphous (lacks crystalline structure). Cleavage: None, exhibits conchoidal fracture (smooth, curved breaks like broken glass). Specific Gravity: 2.3-2.6.

Formation Process: Rapid cooling of viscous, silica-rich (felsic) lava from volcanoes, which prevents atomic crystallization. The lava solidifies so quickly that individual mineral crystals do not have time to grow. Geological Age: Varies widely, found in geologically young volcanic regions, often Cenozoic to recent.

Industry: Used in surgical scalpels due to its ability to be flaked to an extremely sharp edge (sharper than steel). Jewelry: Polished into cabochons, beads, and carved into ornaments. Construction: Rarely used in construction, but historically used for tools and weapons due to its sharp edges. Historically significant for prehistoric tools (arrowheads, knives, scrapers) and weapons.

Obsidian forms most commonly from rhyolitic lavas, which are high in silica and relatively low in iron and magnesium. The dark color is typically due to scattered iron oxides. Snowflake obsidian contains spherulites of cristobalite (a silica mineral). Apache Tears are small, rounded nodules of obsidian. 'Rainbow obsidian' exhibits iridescent sheens due to tiny inclusions of magnetite.

How to Identify in the Field: Look for its distinct glassy luster, black color (though other colors exist), conchoidal fracture, and sharp edges. It's often found in areas with recent volcanic activity. Common Locations: Found globally in areas of past and present volcanic activity, including the United States (Oregon, California, Arizona, New Mexico, Idaho, Wyoming), Mexico, Iceland, Japan, Ecuador, and parts of the Mediterranean. Tips for Collectors: Be aware of its very sharp edges. It can be found as flows, domes, and small nodular pieces. Look for it near rhyolitic volcanic outcrops.