Igneous
River Basalt (Cobble)
Mafic Igneous Rock (Fine-grained Basalt)
Hardness: 5-6 (Mohs); Color: Dark gray, charcoal, or greenish-black; Luster: Dull to earthy (unless polished or wet); Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0.
- Hardness
- 5-6 (Mohs)
- Color
- Dark gray, charcoal, or greenish-black
- Luster
- Dull to earthy (unless polished or wet)
Identified More igneous →
Explore River Basalt (Cobble) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5-6 (Mohs); Color: Dark gray, charcoal, or greenish-black; Luster: Dull to earthy (unless polished or wet); Crystal Structure: Aphanitic (fine-grained); 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 tumbled and rounded by hydraulic action in a river or stream over hundreds to thousands of years.
Uses & applications
Commonly used as crushed stone in construction, road base, and railroad ballast. Smooth river stones are used in landscaping, decorative aquariums, and hot stone 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 extensively on the Moon and Mars.
Field identification & locations
Identify in the field by its high density/weight, dark color, and lack of visible crystals to the naked eye. Look for smooth, rounded edges and a waxy appearance when wet. Very common in volcanic regions and riverbeds.
