igneous
Basalt Pebble
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6-7 Mohs scale. Color: Dark gray, charcoal, to black. Luster: Dull to satin or waxy when river-worn/tumbled. Structure: Fine-grained (aphanitic) microcrystalline texture composed mainly of plagioclase feldspar and pyroxene.
- Hardness
- 6-7 Mohs scale
- Color
- Dark gray, charcoal, to black
- Luster
- Dull to satin or waxy when river-worn/tumbled
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Physical properties
Hardness: 6-7 Mohs scale. Color: Dark gray, charcoal, to black. Luster: Dull to satin or waxy when river-worn/tumbled. Structure: Fine-grained (aphanitic) microcrystalline texture composed mainly of plagioclase feldspar and pyroxene.
Formation & geological history
Formed from the rapid cooling of low-viscosity basaltic lava at or near Earth's surface. Water erosion in rivers, streams, or ocean beaches smoothed and rounded this specimen into a pebble over time.
Uses & applications
Commonly used as crushed stone in construction, road base, concrete aggregate, landscaping gravel, and as a popular souvenir pebble.
Geological facts
Basalt is the most common rock type in Earth's oceanic crust and covers vast areas of the Pacific Northwest, including the Columbia River Basalt Group near Oregon where this photo was taken. Basalt is also abundant on the Moon and Mars.
Field identification & locations
Identified in the field by its dark color, fine-grained texture, heaviness, and lack of visible large quartz crystals. Very common along Pacific Northwest beaches and riverbeds.
