Basalt Pebble

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

Formed from the rapid cooling of low-viscosity, magnesium- and iron-rich lava flows at or near the Earth's surface. Geological age varies widely depending on the deposit.

Used extensively in construction as crushed stone for road bases, concrete aggregate, railroad ballast, and sometimes in dimension stone and countertops.

Basalt is the most common rock type in the Earth's crust underlying the ocean basins and also forms massive volcanic structures like the Giant's Causeway in Northern Ireland and the Columbia River Plateaus.

To identify in the field, look for a very fine-grained, dark-colored heavy rock that may show vesicular textures (gas cavities) or slight weathering to a brownish hue. Commonly found in volcanic regions and riverbeds where weathering has rounded it into pebbles.