Basalt River Cobble

Hardness: 5-6 (Mohs scale), Color: Dark gray to black (weathered to light gray), Luster: Dull or earthy, Crystal structure: Aphanitic (fine-grained), Cleavage: None, Specific gravity: 2.8-3.0.

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. High-energy water transport in rivers or coastlines has rounded this specific specimen over hundreds to thousands of years.

Usually used as aggregate in construction, road base, or railroad ballast. Decorative landscaping and aquarium stone. Polished basalt is sometimes used in jewelry or as 'hot stones' for massage therapy.

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.

Identify by its heavy weight relative to size and very fine-grained texture. It often shows thin white veins of quartz or calcite filling small fractures. Commonly found in volcanic regions and riverbeds downstream from volcanic mountains.