igneous
Basalt with Phenocrysts
Porphyritic Basalt
Hardness: 6.0; Color: dark grey to black matrix with white/clear crystals; Luster: dull to vitreous; Crystal structure: fine-grained (aphanitic) matrix with visible large crystals (phenocrysts); Cleavage: none to poor; Specific gravity: approx. 2.9-3.1
- Hardness
- 6
- Color
- dark grey to black matrix with white/clear crystals
- Luster
- dull to vitreous
Identified More igneous →
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Physical properties
Hardness: 6.0; Color: dark grey to black matrix with white/clear crystals; Luster: dull to vitreous; Crystal structure: fine-grained (aphanitic) matrix with visible large crystals (phenocrysts); Cleavage: none to poor; Specific gravity: approx. 2.9-3.1
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The larger white crystals formed slowly underground before the final eruption.
Uses & applications
Commonly used as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality specimens are used in decorative landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars.
Field identification & locations
Look for a heavy, dark, fine-grained rock with visible speckles of lighter minerals. It is often found in volcanic regions or as river-worn pebbles in areas with volcanic history.
