igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock)
Hardness: 5-6 (Mohs scale); Color: Dark grey to black matrix with white or green spots; Luster: Dull (matrix) to vitreous (fillings); Crystal structure: Aphanitic (fine-grained) with secondary mineral fillings; Cleavage: None; Specific Gravity: 2.8-3.0
- Hardness
- 5-6 (Mohs scale)
- Color
- Dark grey to black matrix with white or green spots
- Luster
- Dull (matrix) to vitreous (fillings)
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark grey to black matrix with white or green spots; Luster: Dull (matrix) to vitreous (fillings); Crystal structure: Aphanitic (fine-grained) with secondary mineral fillings; Cleavage: None; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of basaltic lava. The white spots (amygdules) formed when minerals such as quartz, calcite, or zeolites precipitated from groundwater into gas bubbles (vesicles) trapped in the cooling lava.
Uses & applications
Used primarily as crushed stone for road construction and concrete aggregate. Decorative specimens are valued by amateur collectors and for educational purposes.
Geological facts
The term 'amygdule' comes from the Greek word for 'almond,' referring to the shape of the mineral fillings. These rocks are common in volcanic regions like the Lake Superior area where they often contain native copper.
Field identification & locations
Identify by looking for a dark, heavy rock peppered with small, light-colored mineral 'dots.' Common in basaltic lava flows and glacial till near volcanic provinces.
