igneous
Basalt (Porphyritic)
Porphyritic Basalt
Hardness: 5-6 (Mohs); Color: dark green to black with white or tan inclusions; Luster: dull to earthy (when dry); Structure: fine-grained (aphanitic) groundmass with visible crystals (phenocrysts); Specific Gravity: 2.8–3.0
- Hardness
- 5-6 (Mohs)
- Color
- dark green to black with white or tan inclusions
- Luster
- dull to earthy (when dry)
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Physical properties
Hardness: 5-6 (Mohs); Color: dark green to black with white or tan inclusions; Luster: dull to earthy (when dry); Structure: fine-grained (aphanitic) groundmass with visible crystals (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 visible white spots (phenocrysts) formed at depth before the final eruption. Found in volcanic regions worldwide.
Uses & applications
Commonly used as crushed stone for aggregate in construction, road base, and railroad ballast. Compact, water-worn specimens are often used in landscaping or as decorative river rocks.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The white spots in this specimen are likely plagioclase feldspar crystals that grew slowly underground before the rest of the lava cooled quickly.
Field identification & locations
Identify in the field by its dark color, heavy weight (density), and lack of visible quartz. This specimen is smoothed by water erosion, likely from a river or beach. Found in volcanic provinces like the Pacific Northwest or oceanic islands.
