igneous
Basalt Porphyry
Porphyritic Basalt
Hardness: 6-7 on Mohs scale. Color: Dark grey to charcoal matrix with white/light-grey phenocrysts. Luster: Dull to earthy (weathered surface). Structure: Aphanitic matrix with visible crystals (porphyritic texture).
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to earthy (weathered surface)
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark grey to charcoal matrix with white/light-grey phenocrysts. Luster: Dull to earthy (weathered surface). Structure: Aphanitic matrix with visible crystals (porphyritic texture).
Formation & geological history
Formed from volcanic activity where magma cooled in two stages: slowly at depth (forming the larger white crystals) and then rapidly at or near the surface (forming the fine-grained grey matrix). Often associated with Cenozoic to recent volcanic flows.
Uses & applications
Commonly used as crushed stone for road construction, railroad ballast, and concrete aggregate. Porphyritic varieties are sometimes used for decorative landscaping or as garden stones due to their speckled appearance.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple, originally referring to a specific purple-red volcanic rock prized by Romans. Basalt is the most common rock type in the Earth's crust, covering most of the ocean floor.
Field identification & locations
Identify by looking for distinct, isolated crystals (phenocrysts, likely plagioclase feldspar) embedded in a much finer-grained, darker volcanic matrix. This specific specimen is a water-worn or weathered pebble likely found in a riverbed or coastal area.
