igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock)
Hardness: 5-6 (Mohs); Color: Gray to dark greenish-gray; Luster: Dull/Earthy; Structure: Vesicular/Amygdaloidal (filled cavities); Cleavage: None; Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs)
- Color
- Gray to dark greenish-gray
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Gray to dark greenish-gray; Luster: Dull/Earthy; Structure: Vesicular/Amygdaloidal (filled cavities); Cleavage: None; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from rapidly cooling lava flows where gas bubbles (vesicles) were trapped. Over geological time, secondary minerals like quartz, calcite, or zeolites precipitated from groundwater to fill these holes.
Uses & applications
Primarily decorative as 'lucky stones' or pocket stones; larger deposits are used for road aggregate or construction riprap.
Geological facts
The term 'amygdule' comes from the Latin word for almond, referring to the almond-shaped mineral fillings. These are common in the Lake Superior region and often contain native copper.
Field identification & locations
Identify by looking for a dark, fine-grained matrix containing small circular or oval pits that are filled with lighter-colored minerals. Found in volcanic terrain and glacial till.
