igneous
River Pebble (Basalt or Graywacke)
Mafic Igneous Rock (Basalt)
Hardness: 5-6 (Mohs); Color: Dark gray to black with weathered brownish patches; Luster: Dull/Matte; Structure: Fine-grained aphanitic; Cleavage: None (conchoidal fracture); Shape: Sub-rounded due to fluvial erosion.
- Hardness
- 5-6 (Mohs)
- Color
- Dark gray to black with weathered brownish patches
- Luster
- Dull/Matte
Identified More igneous →
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark gray to black with weathered brownish patches; Luster: Dull/Matte; Structure: Fine-grained aphanitic; Cleavage: None (conchoidal fracture); Shape: Sub-rounded due to fluvial erosion.
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 further shaped by water transport in a river or stream environment.
Uses & applications
Commonly used as construction aggregate, road base, decorative landscaping stone, and in the production of asphalt and concrete.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. When found as rounded pebbles, they are often referred to as 'river rocks'.
Field identification & locations
Identify in the field by its dark color, fine grain size (crystals not visible to the naked eye), and heavy weight for its size. Commonly found in riverbeds, glacial deposits, and volcanic regions.
