Basalt

Commonly dark gray to black, fine-grained (aphanitic) texture due to rapid cooling. Can be vesicular (contain gas bubbles) or amygdaloidal (vesicles filled with secondary minerals). Hardness typically 6 (Mohs). Luster is dull to sub-metallic. Crystal structure is generally invisible to the naked eye; microscopic minerals include plagioclase feldspar, pyroxene, and olivine.

Formed from the rapid cooling of lava exposed at or very near the surface of the Earth or on the ocean floor. It is the most common rock type in the Earth's crust by volume, forming much of the ocean crust as well as large continental flood basalts and volcanic islands. Geologically, it forms continuously and can be as old as billions of years or as young as today from active volcanoes.

Used extensively in construction as crushed aggregate for road base, concrete, and asphalt. Also used in railway ballast, filter stone, and seawalls. Basalt fibers are used in various composite materials. Polished basalt is sometimes used as a decorative stone.

Basalt is the primary component of the Moon's maria, the dark plains visible from Earth. Its rapid solidification often leads to the formation of columnar jointing, such as the famous Giant's Causeway in Northern Ireland or Devil's Tower in Wyoming. It is the extrusive equivalent of gabbro.

In the field, look for a dark, dense, fine-grained rock. It often forms flows, dikes, and sills. If broken, it can have a conchoidal fracture. Common in areas of past or present volcanic activity: Hawaiian Islands, Iceland, along oceanic ridges, and flood basalt provinces like the Deccan Traps (India) and Columbia River Basalt Group (USA). Identifying features include its dark color, fine grain, and sometimes vesicular or columnar structure. It is typically heavy for its size.