Basalt (River Pebble)

Hardness: 6-7 Mohs; Color: Dark grey to black; Luster: Dull/Matte; Crystal structure: Aphanitic (fine-grained); 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. This specific specimen has been smooth-rounded by water erosion in a river or coastal environment.

Commonly used in construction (aggregate), high-end landscaping, massage therapy (hot stones), and as a base material for road beds.

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found extensively on the Moon and Mars.

Identify in the field by its heavy weight (high density), dark color, and lack of visible crystals. Commonly found in volcanic regions or as rounded pebbles in riverbeds and on beaches.