igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (extrusive volcanic rock with secondary mineral infilling)
Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-black matrix with light green or white spots; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Vesicular texture filled with secondary minerals; Cleavage: None (conchoidal fracture).
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull/earthy matrix with vitreous inclusions
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-black matrix with light green or white spots; Luster: Dull/earthy matrix with vitreous inclusions; Structure: Vesicular texture filled with secondary minerals; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the cooling of gas-rich lava flows. As gas bubbles (vesicles) are trapped in the cooling basalt, they later become filled with secondary minerals like epidote, quartz, or zeolites through groundwater infiltration. This specimen is likely from the Keweenawan age (approx. 1.1 billion years ago).
Uses & applications
Commonly used as decorative garden stones, for aquarium decor, or as cabochons in lapidary work. In smaller sizes, it is a popular specimen for educational geology kits.
Geological facts
The word 'amygdaloidal' comes from the Greek 'amygdala,' meaning almond, referring to the almond-shaped mineral deposits. This specific greenish variety is often associated with the 'Greenstone' (Chlorastrolite) found in Michigan's Lake Superior region.
Field identification & locations
Identify in the field by looking for dark, heavy volcanic rocks with distinct 'polka-dot' mineral fillings. Common in volcanic regions and glacial drifts, particularly around the Great Lakes and Pacific Northwest. Collectors often look for ones where the 'eyes' are bright green epidote.
