Basalt

Aphanitic (fine-grained) texture, Mohs hardness 6, dark gray to black color, dull to sub-metallic luster, high specific gravity (approx 2.8-3.0), no cleavage.

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the surface of a terrestrial planet or a moon. It is the most common volcanic rock on Earth and makes up most of the ocean floor.

Extensively used in construction for aggregate, road base, railroad ballast, and high-performance asphalt. Tumbled specimens are used in stone massage therapy.

Basalt is equivalent in composition to gabbro, but gabbro is intrusive (cooled slowly underground) while basalt is extrusive. It is the most common rock type in the Earth's crust.

Identified by its heavy weight relative to size and very fine-grained dark appearance. Look for it in volcanic fields, riverbeds draining volcanic areas, or as common landscaping gravel.