igneous
Amygdaloidal Basalt
Amygdaloidal Basalt containing Zeolite or Quartz (SiO2/Silicates)
Hardness: 5-7 (Mohs); Color: Dark grey/black matrix with white or light blue circular inclusions; Luster: Dull/earthy matrix with vitreous/pearly vesicles; Structure: Aphanitic with vesicular/amygdaloidal texture.
- Hardness
- 5-7 (Mohs)
- Luster
- Dull/earthy matrix with vitreous/pearly vesicles
Identified More igneous →
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Physical properties
Hardness: 5-7 (Mohs); Color: Dark grey/black matrix with white or light blue circular inclusions; Luster: Dull/earthy matrix with vitreous/pearly vesicles; Structure: Aphanitic with vesicular/amygdaloidal texture.
Formation & geological history
Formed from the cooling of mafic lava flows. Gasses escaping the lava create bubbles (vesicles). Over millions of years, mineral-rich groundwater deposits minerals like quartz, calcite, or zeolites into these holes, creating 'amygdules'.
Uses & applications
Common basalt is used in road construction and concrete aggregate. Amygdaloidal specimens are primarily for geological study and mineral collecting. Polished specimens are occasionally used in lapidary work.
Geological facts
The word 'amygdule' is derived from the Greek word 'amygdala', meaning almond, referring to the almond-shaped mineral deposits often found in these volcanic rocks.
Field identification & locations
Identify by looking for a dark, heavy, fine-grained volcanic rock containing distinct, round 'spots' of a different material. Common in volcanic regions like the Lake Superior Basin, Iceland, and the Deccan Traps.
