igneous
Porphyritic Basalt
Amygdaloidal Porphyritic Basalt
Hardness: 5-6 (Mohs); Color: Dark grey to black matrix with white/tan crystals; Luster: Dull to sub-vitreous; Structure: Aphanitic matrix with phenocrysts; Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs)
- Color
- Dark grey to black matrix with white/tan crystals
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to black matrix with white/tan crystals; Luster: Dull to sub-vitreous; Structure: Aphanitic matrix with phenocrysts; 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 large crystals (phenocrysts) grew slowly underground before the final eruption. Common in volcanic island arcs and oceanic crust.
Uses & applications
Used primarily in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Attractive specimens are used in landscaping or by amateur collectors.
Geological facts
Basalt is the most common rock type in the Earth's crust and covers most of the ocean floor. The 'spots' seen here are likely phenocrysts of plagioclase feldspar or possibly amygdules (gas bubbles filled with secondary minerals like quartz or calcite).
Field identification & locations
Identify by its dark, heavy fine-grained matrix and distinct lighter-colored crystal inclusions. Often found in areas with historic volcanic activity like the Pacific Northwest or Rift Valleys.
