Basalt

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to vitreous; Crystal Structure: Aphanitic (fine-grained), sometimes porphyritic with larger phenocrysts; Cleavage: None; Specific Gravity: 2.8-3.0

Basalt forms from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the Earth's surface. It is a common volcanic rock and can be found in lava flows, shield volcanoes, oceanic crust, and dikes. Most basalts are Mesozoic to Cenozoic in age, but older basalts exist.

Basalt is widely used in construction (e.g., aggregate for roads, concrete, asphalt), as paving stones, and in the production of basalt fibers for textiles and composites. Due to its durability and resistance to weathering, it is also used for building facades and monuments.

Basalt is the most common rock type in the Earth's crust, forming the bulk of the ocean floor. The giant's causeway in Northern Ireland and the Deccan Traps in India are famous examples of basaltic formations. It is also found on the Moon, Mars, and other terrestrial planets.

In the field, basalt can be identified by its dark color, fine-grained texture (though vesicles and phenocrysts may be present), and relative heaviness. It often has a rough, scoriaceous or vesicular texture on weathered surfaces due to gas bubbles. It is commonly found in volcanic regions, oceanic islands (like Hawaii), mid-ocean ridges, and continental flood basalt provinces. Collectors often seek out specimens with interesting textures or mineral inclusions.