igneous
Basalt Porphyry
Porphyritic Basalt
Hardness: 6-7 (Mohs scale); Color: Dark grey to black matrix with tan or light-colored phenocrysts; Luster: Dull to earthy (weathered surface); Texture: Aphanitic matrix with visible crystals (phenocrysts); Cleavage: None visible in hand specimen.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to earthy (weathered surface)
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to black matrix with tan or light-colored phenocrysts; Luster: Dull to earthy (weathered surface); Texture: Aphanitic matrix with visible crystals (phenocrysts); Cleavage: None visible in hand specimen.
Formation & geological history
Formed from the rapid cooling of basaltic lava at or near the Earth's surface, where larger crystals (phenocrysts) had already begun to form while the magma was still underground. This specimen shows significant rounding from water erosion (fluvial transport).
Uses & applications
Commonly used in crushed stone for road construction, concrete aggregate, and railroad ballast. Due to its durability, it is also used in landscaping and as a pocket stone for collectors.
Geological facts
The two-stage cooling process that creates porphyritic rocks is a window into the plumbing system of volcanoes, showing how minerals crystallize at different depths and temperatures.
Field identification & locations
Identify by looking for distinct, larger mineral grains embedded in a much finer-grained, dark background. Commonly found in riverbeds, glacial deposits, and near volcanic regions. This specific piece is a river-worn pebble.
