igneous
Basalt (River Pebble)
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 on Mohs scale. Color: Dark grey to black. Luster: Dull or earthy. Structure: Aphanitic (fine-grained), sometimes vesicular (having small gas bubbles). Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black
- Luster
- Dull or earthy
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark grey to black. Luster: Dull or earthy. Structure: Aphanitic (fine-grained), sometimes vesicular (having small gas bubbles). 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 rounded into a 'pebble' shape through water erosion in a river or coastal environment over thousands of years.
Uses & applications
In bulk, it is used for road base, concrete aggregate, and railroad ballast. Smooth river stones are popular in landscaping, aquarium decor, and massage therapy (hot stone massage).
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 and Mars. When basalt cools in thick layers, it often forms striking hexagonal columns, such as at the Giant's Causeway.
Field identification & locations
Identify in the field by its dark color, heavy weight (dense), and fine-grained texture where individual crystals are too small to see with the naked eye. Commonly found on beaches, riverbeds, and near volcanic regions like the Pacific Northwest or Hawaii.
