Basalt (Stream-worn Cobble)

Physical Properties

Hardness: 6.0; Color: Dark grey to dull greenish-grey (weathered); Luster: Dull/Earthy (weathered); Structure: Fine-grained (Aphanitic); Specific Gravity: 2.8 - 3.0.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specific specimen has been significantly rounded and smoothed by fluvial (water) or glacial transport.

Uses & Applications

Crushed basalt is used extensively in construction for road base, concrete aggregate, and railroad ballast. Unweathered pieces are used as dimension stone or for river rock landscaping.

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 extensively on the Moon and Mars.

Field Identification & Locations

Identify in the field by its heavy weight (density), fine grain size, and dark interior when chipped. It is commonly found in volcanic regions or as 'erratics' in glacial till and riverbeds. This specimen shows a typical weathered 'rind'.

Identified on: 4/25/2026

Mode: Standard