igneous
Basalt (River-worn Pebble)
Aphanitic Mafic Igneous Rock
Hardness: 6 on Mohs scale; Color: Dark grey to dull black; Luster: Dull/Sub-metallic (when wet); Crystal structure: Aphanitic (fine-grained, crystals not visible to eye); Cleavage: None/Conchoidal fracture; Specific gravity: 2.8-3.0.
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to dull black
- Luster
- Dull/Sub-metallic (when wet)
Identified More igneous →
Explore Basalt (River-worn Pebble) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to dull black; Luster: Dull/Sub-metallic (when wet); Crystal structure: Aphanitic (fine-grained, crystals not visible to eye); Cleavage: None/Conchoidal fracture; 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 chemically weathered and physically rounded by fluvial (river) or marine transport.
Uses & applications
Basalt is used extensively in construction as aggregate for roads, railroad ballast, and in concrete. Polished pebbles are often used in landscaping, decorative stone displays, 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 on the Moon, Mars, and Venus. Large basaltic flows are known as 'traps'.
Field identification & locations
Identify in the field by its dark color, heavy weight for its size (high density), and lack of visible crystals. It often develops a thin, lighter-colored weathering rind. Commonly found near volcanic sites or in riverbeds draining volcanic regions.
