igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic Volcanic Rock)
Hardness: 5-6 Mohs. Color: Dark gray to black matrix with white or green mineral infills. Luster: Dull (matrix) to vitreous (vesicle infills). Structure: Porphyritic/Vesicular. Specific Gravity: 2.8-3.0.
- Hardness
- 5-6 Mohs
- Luster
- Dull (matrix) to vitreous (vesicle infills)
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Physical properties
Hardness: 5-6 Mohs. Color: Dark gray to black matrix with white or green mineral infills. Luster: Dull (matrix) to vitreous (vesicle infills). Structure: Porphyritic/Vesicular. Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the cooling of gas-rich lava flows. As gas bubbles (vesicles) are trapped in solidifying lava, they are later filled by secondary minerals like quartz, calcite, or zeolites through groundwater precipitation. Most common in Precambrian and Cenozoic volcanic regions.
Uses & applications
Used primarily as construction aggregate and road base. Polished specimens are often sought after by geological collectors for their unique "polka dot" appearance. Some higher-quality amygdaloids containing agate or copper are used in decorative jewelry.
Geological facts
In the Lake Superior region, these rocks are famous for containing native copper or semi-precious agates within the small almond-shaped pockets (amygdules). The term 'amygdule' comes from the Greek word for 'almond'.
Field identification & locations
Identify in the field by looking for a dense, dark volcanic matrix containing rounded, light-colored inclusions that are not grain-like. Commonly found in basaltic lava fields, particularly in the Keweenaw Peninsula of Michigan, the Pacific Northwest, and Iceland.
