igneous
Basalt Pebble
Extrusive mafic igneous rock; primarily (Ca, Na)(Mg, Fe, Al)Si2O6 and (Ca, Na)AlSi3O8
Hardness: 5-6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None; Specific gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the earth's surface. This specific specimen has been rounded and smoothed by water erosion (fluvial or glacial action) during the Cenozoic era.
Uses & applications
Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Smooth basalt pebbles are popular in landscaping and decorative masonry.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up the majority 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), dark uniform color, and fine grain. Commonly found in volcanic regions, riverbeds, and coastal areas downstream from volcanic flows.
