igneous
Amygdaloidal Basalt
Vesicular-Amygdaloidal Basalt (Mafic Extrusive)
Hardness: 5-6 (Mohs); Color: Dark grey, reddish-brown, or purple-maroon groundmass with white, green, or pink mineral spots; Luster: Dull/Earthy (groundmass), Vitreous (fillings); Structure: Aphanitic with rounded amygdules.
- Hardness
- 5-6 (Mohs)
- Luster
- Dull/Earthy (groundmass), Vitreous (fillings)
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark grey, reddish-brown, or purple-maroon groundmass with white, green, or pink mineral spots; Luster: Dull/Earthy (groundmass), Vitreous (fillings); Structure: Aphanitic with rounded amygdules.
Formation & geological history
Formed from gas bubbles trapped in cooling lava (vesicles). Later, mineral-rich groundwater flows through the rock, depositing minerals like quartz, calcite, or zeolites in the voids. Commonly found in Mesozoic or Precambrian volcanic flood deposits.
Uses & applications
Commonly used as high-grade road base or concrete aggregate. Polished specimens are used in the lapidary arts for jewelry, particularly those from the Lake Superior region containing copper or agates.
Geological facts
The spots in the rock are called amygdules, derived from the Greek word 'amygdalē' meaning almond. In the Keweenaw Peninsula, these cavities are often filled with native copper, creating some of the richest copper deposits in history.
Field identification & locations
Identify by looking for a fine-grained, dark igneous matrix containing distinct, rounded 'eye-like' structures filled with secondary minerals. Found in basaltic lava flow areas.
