Basalt Pebble

Physical Properties

Hardness: 6 on Mohs scale; Color: Dark grey to black, with light brown oxidation patches; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic (fine-grained), sometimes containing microscopic plagioclase feldspar and pyroxene; Specific Gravity: 2.8-3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. This specific specimen shows water-worn rounding, indicating it was smoothed over time in a river, stream, or beach environment. Basalt forms in diverse geological ages, from the Precambrian to modern eruptions.

Uses & Applications

Used extensively in the construction industry as crushed stone for road base, railroad ballast, and concrete aggregate. Polished pebbles are used in landscaping and aquariums.

Geological Facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon, Mars, and Venus. The dark 'seas' or maria on the Moon are actually vast basaltic plains.

Field Identification & Locations

Identified by its dark color, heavy weight (relative to size), and fine-grained texture. To identify in the field, look for dark, dense rocks in volcanic regions or as rounded cobbles in waterways. It will leave a grey to black streak if scratched on porcelain.

Identified on: 5/2/2026

Mode: Standard