igneous
Porphyritic Basalt
Amygdaloidal Porphyritic Basalt
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with light green/white phenocrysts (often plagioclase or olivine), Luster: Dull to earthy when dry, greasy/vitreous when wet, Structure: Porphyritic (large crystals in a fine-grained matrix), Specific Gravity: 2.8–3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with light green/white phenocrysts (often plagioclase or olivine), Luster: Dull to earthy when dry, greasy/vitreous when wet, Structure: Porphyritic (large crystals in a fine-grained matrix), Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. The larger crystals (phenocrysts) formed slowly underground before being erupted. These specimens are common in basaltic flows like the Keweenawan Rift in North America (approx. 1.1 billion years old).
Uses & applications
Primarily used as decorative stones in landscaping, river rock, or for tumbled collectors items. Large-scale basalt is used for road base and concrete aggregate.
Geological facts
This specimen appears to be water-worn, likely a 'river rock' or 'beach pebble'. When basalt contains large visible crystals like this, it is called a 'porphyry', a term derived from the Greek word for purple, though it now refers to the texture rather than color.
Field identification & locations
Identify in the field by looking for a dark, heavy, fine-grained rock with distinct, lighter-colored rectangular or dot-like crystal inclusions. Often found in volcanic regions, Great Lakes beaches, or riverbeds downstream from basaltic outcrops.
