igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic volcanic rock)
Hardness: 5-6 (Mohs scale); Color: Dark brown or gray groundmass with white, secondary mineral inclusions; Luster: Dull/earthy matrix with pearly to vitreous amygdules; Crystal structure: Aphanitic (fine-grained); Cleavage: None/cluttersome.
- Hardness
- 5-6 (Mohs scale)
- Luster
- Dull/earthy matrix with pearly to vitreous amygdules
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Physical properties
Hardness: 5-6 (Mohs scale); Color: Dark brown or gray groundmass with white, secondary mineral inclusions; Luster: Dull/earthy matrix with pearly to vitreous amygdules; Crystal structure: Aphanitic (fine-grained); Cleavage: None/cluttersome.
Formation & geological history
Formed from the rapid cooling of basaltic lava. The "amygdules" (the white spots) form when gas bubbles (vesicles) in the cooling lava are later filled by secondary minerals like quartz, calcite, or zeolites through groundwater infiltration over millions of years.
Uses & applications
Primarily used as construction aggregate or crushed stone. Highly aesthetic specimens with semi-precious fillings (like agate) are used for lapidary work and collectors.
Geological facts
This rock is famous in the Lake Superior region of North America, where the amygdules can sometimes contain native copper instead of common minerals. The name 'amygdule' comes from the Greek word for 'almond'.
Field identification & locations
Identify by looking for a dark, dense matrix filled with 'pockmarks' or 'spots' of lighter minerals. Often found in volcanic fields, riverbeds, and glacial till. Collectors look for colorful fillings like agate or chlorite.
