igneous
Amygdaloidal Basalt
Amygdaloidal Mafic Volcanic Rock (Basalt)
Hardness: 5-6 (Mohs); Color: Dark grey to reddish-brown groundmass with lighter tan or white circular infillings; Luster: Dull or earthy; Crystal Structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
- Hardness
- 5-6 (Mohs)
- Luster
- Dull or earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey to reddish-brown groundmass with lighter tan or white circular infillings; Luster: Dull or earthy; Crystal Structure: Aphanitic (fine-grained); Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the cooling of gas-rich lava. As the lava cools, gas bubbles (vesicles) are trapped. Over thousands to millions of years, mineral-rich groundwater flows through these holes, depositing secondary minerals. These filled vesicles are called 'amygdules'.
Uses & applications
Commonly used as crushed stone in road construction or railroad ballast. Specimens with high-quality mineral infillings (like agate or zeolites) are popular among mineral collectors and for lapidary work.
Geological facts
The term 'amygdule' comes from the Greek word 'amygdala', meaning almond, because of the almond-like shape of the mineral fillings. This rock is the primary source of Lake Superior Agates and native copper in Michigan's Keweenaw Peninsula.
Field identification & locations
Identify in the field by looking for a dark, heavy volcanic rock riddled with small circular or oval pits that are either empty (vesicles) or filled with a different colored mineral (amygdules). Common in volcanic regions such as the Pacific Northwest, Lake Superior region, and Iceland.
