Basalt

Hardness of 6 on the Mohs scale, dark gray to black color, aphanitic (fine-grained) texture, dull to sub-metallic luster, specific gravity around 2.8-3.0.

Formed from the rapid cooling of low-viscosity basaltic lava exposed at or very near the Earth's surface, typically associated with volcanic eruptions, oceanic crust formation, and hot spots.

Used extensively in construction as crushed stone for road bases, concrete aggregate, and railroad ballast, as well as in the production of basalt fiber.

Basalt is the most common rock type in the Earth's crust underlying the world's oceans, and it is also found on the Moon (forming the lunar maria) and Mars.

Identify in the field by its dark color, heavy weight, and very fine-grained matrix where individual crystals are typically microscopic; common globally in volcanic regions.