igneous
Basalt Pebble
Porphyritic Basalt
Hardness: 6 on Mohs scale; Color: Dark grey to black with light-colored phenocryst specks; Luster: Dull/earthy; Texture: Fine-grained (aphanitic) matrix with scattered small crystals; Density: Heavy (SG 2.8 - 3.0).
- Hardness
- 6 on Mohs scale
- Color
- Dark grey to black with light-colored phenocryst specks
- Luster
- Dull/earthy
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Physical properties
Hardness: 6 on Mohs scale; Color: Dark grey to black with light-colored phenocryst specks; Luster: Dull/earthy; Texture: Fine-grained (aphanitic) matrix with scattered small crystals; Density: Heavy (SG 2.8 - 3.0).
Formation & geological history
Formed by the rapid cooling of low-viscosity mafic lava at or near the Earth's surface. The smooth, rounded pebble shape was created by stream or coastal wave erosion over extended time.
Uses & applications
Basalt is used extensively in construction as crushed aggregate, road base, asphalt filler, and railway ballast. Rounded river or beach pebbles are commonly used in landscaping and aquascapes.
Geological facts
Basalt is the most common rock type in the Earth's oceanic crust and also covers vast areas of the Moon (the dark lunar maria) and Mars.
Field identification & locations
Identified by its dark charcoal-to-black color, dense weight, fine-grained texture, and small embedded plagioclase feldspar or pyroxene phenocrysts. Highly consistent with glacial drift or river gravels found across the UK Midlands region (e.g., Shropshire/West Midlands near 52.4°N, 2.2°W).
