igneous
Amygdaloidal Basalt
Basalt with secondary mineral infilling (commonly Zeolite, Calcite, or Quartz)
Hardness: 5-6 (matrix); Color: Dark grey to black matrix with white or green spots (amygdules); Luster: Dull matrix with vitreous/pearly inclusions; Texture: Aphanitic with vesicles; Crystal Structure: Microcrystalline.
- Hardness
- 5-6 (matrix)
- Luster
- Dull matrix with vitreous/pearly inclusions
Identified More igneous →
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Physical properties
Hardness: 5-6 (matrix); Color: Dark grey to black matrix with white or green spots (amygdules); Luster: Dull matrix with vitreous/pearly inclusions; Texture: Aphanitic with vesicles; Crystal Structure: Microcrystalline.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. Gas bubbles (vesicles) trapped in the cooling lava are later filled with secondary minerals precipitated from hydrothermal fluids flowing through the rock.
Uses & applications
Used as decorative stones, in landscaping, as road aggregate, and as a popular specimen for educational kits or amateur mineral collecting.
Geological facts
The term 'amygdule' comes from the Greek word 'amygdala', meaning almond, referring to the typical shape of the mineral-filled gas bubbles. Many famous Lake Superior agates began as amygdules in basalt.
Field identification & locations
Identified by the dark, heavy volcanic matrix containing distinct, rounded lighter-colored mineral 'spots'. Commonly found in volcanic regions like the Keweenaw Peninsula (Michigan), Iceland, and parts of India and Brazil.
