igneous
Porphyritic Basalt
Basalt (with plagioclase or k-feldspar phenocrysts)
Hardness: 6-7 Mohs; Color: Dark grey to black matrix with reddish-brown or pink phenocrysts; Luster: Dull/Earthy; Structure: Porphyritic (large crystals in fine-grained groundmass); Specific Gravity: 2.8-3.0
- Hardness
- 6-7 Mohs
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to black matrix with reddish-brown or pink phenocrysts; Luster: Dull/Earthy; Structure: Porphyritic (large crystals in fine-grained groundmass); Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The larger 'spots' (phenocrysts) formed slowly underground before the final eruption. Common in volcanic regions and glacial till.
Uses & applications
Primarily used as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally durable specimens are used in landscaping or as decorative pocket stones.
Geological facts
The 'porphyritic' texture tells a story of two-stage cooling: first cooling slowly deep underground to grow large crystals, then erupting and cooling quickly to create the fine background.
Field identification & locations
Look for a dark, dense, fine-grained rock with distinct, differently colored mineral spots. This specimen appears water-worn, likely found in a riverbed or on a beach.
