igneous
Porphyritic Amygdaloidal Basalt
Amygdaloidal Porphyritic Basalt (Mafic volcanic rock)
Hardness: 6-7 on Mohs scale. Color: Dark olive green to grayish-black groundmass with white to light-colored phenocrysts/inclusions. Luster: Dull to vitreous when wet. Texture: Porphyritic (fine-grained groundmass with larger visible crystals). Cleavage: None visible in hand specimen.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous when wet
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark olive green to grayish-black groundmass with white to light-colored phenocrysts/inclusions. Luster: Dull to vitreous when wet. Texture: Porphyritic (fine-grained groundmass with larger visible crystals). Cleavage: None visible in hand specimen.
Formation & geological history
Formed from the cooling of magnesium and iron-rich lava. The green color often indicates alteration to chlorite or epidote, while the white spots are phenocrysts (often plagioclase feldspar) or amygdules (gas bubbles filled with minerals like quartz, calcite, or zeolites) formed during the Late Pre-Cambrian to Cenozoic eras.
Uses & applications
Used primarily as crushed stone for construction, road base, and concrete aggregate. Polished specimens are used for lapidary work, pocket stones, and decorative landscaping.
Geological facts
Many specimens found along the Great Lakes (like this one) originate from the Midcontinent Rift System, a 1.1 billion-year-old volcanic event. The white inclusions represent minerals that crystallized early or filled bubbles long after the lava hardened.
Field identification & locations
Identify by its dark, heavy weight for its size and characteristic 'polka dot' appearance of contrasting light crystals against a dark green or black matrix. Commonly found on beaches of Lake Superior and in volcanic regions worldwide.
