Basalt

Hardness: 6 (Mohs scale); Color: Dark grey to black; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. It can range in age from modern volcanic flows to billions of years old.

Extensively used in construction as aggregate (base for roads, railroad ballast), as durable paving stones, and sometimes as a component in stonewash textiles or high-temperature insulation mineral wool.

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found abundantly on the Moon, Mars, and Venus, making up the lunar maria.

Identify in the field by its dark color, heavy weight (density), and lack of visible large crystal grains without a lens. Often found in volcanic regions, oceanic islands (like Hawaii), and massive 'flood basalt' plateaus. Collectors look for basalt containing vesicles (gas bubbles) or olivine inclusions.