Basalt

Physical Properties

Hardness: 6 on Mohs scale; Color: Dark grey to black, sometimes greenish; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained), often vesicular (bubbly texture seen as small pits); 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 is a water-worn river cobble, likely millions of years old, smoothed by hydraulic action.

Uses & Applications

Used extensively in construction as aggregate (crushed stone for roads), in asphalt and concrete, and as high-quality decorative river stones for landscaping and massage therapy.

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 and Mars. When cooling slowly under specific conditions, it can form massive hexagonal columns like the Giant's Causeway.

Field Identification & Locations

Identify in the field by its heavy weight (density), dark uniform color, fine grain, and the presence of small 'vesicles' (gas bubble pits) on the surface. Commonly found in volcanic regions or as rounded cobbles in riverbeds.

Identified on: 4/24/2026

Mode: Standard