igneous
Amygdaloidal Basalt
Amygdaloidal Basalt
Hardness (typical): 6 on Mohs scale; Color: dark grey to brownish-purple groundmass with white/light-colored spots; Luster: dull to earthy matrix with vitreous inclusions; Crystal structure: aphanitic; Cleavage: none.
- Luster
- dull to earthy matrix with vitreous inclusions
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Physical properties
Hardness (typical): 6 on Mohs scale; Color: dark grey to brownish-purple groundmass with white/light-colored spots; Luster: dull to earthy matrix with vitreous inclusions; Crystal structure: aphanitic; Cleavage: none.
Formation & geological history
Formed from the cooling of gas-rich lava flows. Voids created by gas bubbles (vesicles) are later filled with secondary minerals like calcite, quartz, or zeolites through groundwater precipitation. Often found in Proterozoic volcanic sequences.
Uses & applications
Used primarily as decorative landscaping stones, aquarium decor, and as a geological study aid. Occasional specimens with semi-precious fillings can be used for cabochons.
Geological facts
The word 'Amygdule' comes from the Greek word for 'almond,' referring to the shape of the mineral-filled cavities. It is very common in the Lake Superior region of North America.
Field identification & locations
Identify in the field by looking for a dark, dense volcanic rock containing distinct almond-shaped spots or 'pores' filled with light minerals. Found in areas of past volcanic activity.
