Igneous
Basalt (River Pebble)
Basalt (Aphanitic Mafic Volcanic Rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull/Matte (earthy); Crystal structure: Fine-grained/Aphanitic (not visible to naked eye); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black
- Luster
- Dull/Matte (earthy)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black; Luster: Dull/Matte (earthy); Crystal structure: Fine-grained/Aphanitic (not visible to naked 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 exposed at or very near the surface of a terrestrial planet or a moon. This specific specimen has been rounded and smoothed by water erosion in a fluvial (river) or coastal environment.
Uses & applications
Commonly used in construction as aggregate (crushed stone), for asphalt pavement, railroad ballast, and in the manufacturing of floor tiles. In landscaping, these smooth pebbles are used for decorative drainage and garden paths.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. It is also extremely common on the Moon (the dark 'Maria' or seas) and Mars.
Field identification & locations
Identify in the field by its high density (heavy for its size), dark uniform color, and lack of visible crystals. It is frequently found as rounded cobbles in riverbeds and on beaches near volcanic regions.
