igneous
Porphyritic Basalt
Porphyritic Basalt (with Plagioclase and Augite phenocrysts)
Hardness: 6.0; Color: Dark grey to greenish-black matrix with lighter phenocrysts; Luster: Dull/Sub-vitreous; Crystal Structure: Aphanitic matrix with visible crystals; Specific Gravity: 2.8-3.0
- Hardness
- 6
- Color
- Dark grey to greenish-black matrix with lighter phenocrysts
- Luster
- Dull/Sub-vitreous
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Physical properties
Hardness: 6.0; Color: Dark grey to greenish-black matrix with lighter phenocrysts; Luster: Dull/Sub-vitreous; Crystal Structure: Aphanitic matrix with visible crystals; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of basaltic lava on the surface, where larger crystals (phenocrysts) grew slowly at depth before the final eruption. Common in volcanic island arcs and seafloor spreading centers.
Uses & applications
Used primarily as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally durable for construction.
Geological facts
The 'two-stage' cooling process creates the distinctive 'dalmatian' or speckled look. Basalt is the most common rock type in the Earth's crust and even makes up much of the Moon's lunar mare.
Field identification & locations
Identify by looking for a fine-grained dark background stone containing distinct, larger individual mineral crystals. Frequently found on beaches or in riverbeds near volcanic regions.
