Igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic silicate with secondary mineral inclusions)
Hardness: 5-6 (base rock), 7 (quartz/chalcedony inclusions); Color: Dull brownish-gray to dark green matrix with circular voids or mineral fillings; Luster: Earthy to sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix.
- Hardness
- 5-6 (base rock), 7 (quartz/chalcedony inclusions)
- Luster
- Earthy to sub-metallic
Identified More igneous →
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Physical properties
Hardness: 5-6 (base rock), 7 (quartz/chalcedony inclusions); Color: Dull brownish-gray to dark green matrix with circular voids or mineral fillings; Luster: Earthy to sub-metallic; Crystal structure: Aphanitic (fine-grained) matrix.
Formation & geological history
Formed from the rapid cooling of mafic lava flows. Gas bubbles (vesicles) are trapped as the lava hardens; later, groundwater deposits secondary minerals like quartz, calcite, or zeolites into these holes, forming the almond-shaped 'amygdules.'
Uses & applications
Commonly used as crushed stone for road construction and concrete aggregate. Attractive specimens are used in decorative stone landscaping or lapidary work (tumbling and polishing).
Geological facts
The word 'amygdaloidal' comes from the Greek word 'amygdale,' meaning almond, referring to the shape of the mineral fillings. These rocks are often the source for Lake Superior Agates.
Field identification & locations
Identify by looking for a dark, dense, fine-grained igneous rock containing distinct, rounded pits or solid mineral 'spots' of a different color. Commonly found in volcanic regions such as the Keweenawan Peninsula of Michigan or the basaltic flows of the Pacific Northwest.
