Basalt

Hardness: 6-7 (Mohs), Color: Typically dark gray to black, sometimes greenish, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic (fine-grained) to porphyritic, Cleavage: None, generally massive, Specific Gravity: 2.7-3.3

Formation Process: Rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. Geological Age: Ranges from very recent (volcanic eruptions) to billions of years old. Environments: Mid-ocean ridges, oceanic islands (hotspots), continental flood basalts, and volcanoes.

Industry/Construction: Crushed basalt is used as aggregate in construction for roads, concrete, and asphalt. Cut basalt is used for flagstones, building blocks, and architectural features. Jewelry: Rarely used in jewelry, but sometimes polished for cabochons. Other: Used in the production of basalt fiber for composites.

Basalt is the most common rock type in the Earth's crust, forming the majority of oceanic crust. It is also found extensively on other planetary bodies, such as the Moon and Mars. Notable formations include the Giant's Causeway in Northern Ireland and the Deccan Traps in India.

Field Identification: Look for a dark, fine-grained, heavy rock. It often has small vesicles (gas bubbles) or larger phenocrysts (crystals) within a fine matrix. It typically forms columnar joints in thick flows. Common Locations: Found globally wherever volcanic activity occurs, including Hawaii, Iceland, the Pacific Northwest of the USA, and large parts of Russia, India, and China. Tips for Collectors: Fresh fractures appear dark and even-grained. Weathered surfaces can be lighter. The presence of olivine or pyroxene phenocrysts can confirm its identity.