Basalt

Hardness: 6 (Mohs), Color: Dark grey to black, Luster: Dull/Matte, Structure: Fine-grained (Aphanitic), Specific Gravity: 2.8-3.0

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. It is the primary rock of the oceanic crust.

Used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Also used as dimension stone and in high-quality floor tiles.

Basalt is the most common rock type in the Earth's crust and also covers large areas of the Moon, Mars, and Venus. On the Moon, Basaltic plains are known as 'Lunar Maria'.

Identify in the field by its dark color, heavy weight compared to other similar-sized rocks, and lack of visible grains to the naked eye. Often found in volcanic regions or coastal areas.