igneous
Amygdaloidal Basalt with Zeolite/Quartz Infill
Amygdaloidal Basalt (Host: (Mg,Fe)2SiO4 / CaAl2Si2O8; Infill: SiO2 or Na2Al2Si3O10·2H2O)
Hardness: 5-6 (Basalt) / 7 (Quartz). Color: Dark gray to black matrix with white or translucent circular inclusions. Luster: Dull/earthy matrix with vitreous (glassy) amygdules. Structure: Vesicular texture filled with secondary minerals.
- Hardness
- 5-6 (Basalt) / 7 (Quartz)
- Luster
- Dull/earthy matrix with vitreous (glassy) amygdules
Identified More igneous →
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Physical properties
Hardness: 5-6 (Basalt) / 7 (Quartz). Color: Dark gray to black matrix with white or translucent circular inclusions. Luster: Dull/earthy matrix with vitreous (glassy) amygdules. Structure: Vesicular texture filled with secondary minerals.
Formation & geological history
Formed from cooling lava flows where gas bubbles (vesicles) were trapped. Over millions of years, mineral-rich groundwater seeped through the porous basalt, depositing minerals like quartz, calcite, or zeolites into the voids. Common in Cenozoic or Mesozoic lava fields.
Uses & applications
Used primarily as decorative specimens for collectors, road aggregate in construction, and occasionally for lapidary work if the amygdules are large enough to be cut into 'Lake Superior Agate' or similar cabochons.
Geological facts
The term 'amygdule' comes from the Latin word 'amygdala,' meaning almond, referring to the almond-shaped mineral deposits. These rocks are key indicators for geologists of ancient volcanic gas content and hydrothermal activity.
Field identification & locations
Identify by looking for a heavy, dark, fine-grained igneous rock with distinct 'polka-stots' of lighter mineral material. Commonly found in the Lake Superior region (USA/Canada), Deccan Traps (India), and Iceland.
