Basalt

Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained) to porphyritic; Specific gravity: 2.8-3.0.

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. Most basalt flows on Earth were formed during the Cenozoic Era.

Extensively used in construction, especially as crushed stone for road base, concrete aggregate, and railroad ballast. Also used as a raw material for stone wool insulation and occasionally as a decorative building stone.

Basalt is the most common rock type in the Earth's crust; most of the ocean floor is composed of basalt. It is also common on the Moon and Mars, forming the dark plains known as lunar maria.

Identified in the field by its dark color, heavy weight (density), and lack of visible large crystal grains without magnification. It often exhibits columnar jointing in large outcrops. Common in volcanic regions like Hawaii, the Pacific Northwest (Columbia River Basalts), and Iceland.