igneous
Amygdaloidal Basalt
Extrusive Igneous Rock with Amygdules
Hardness: 6 on Mohs scale; Color: Reddish-brown matrix with lighter spots (amygdules filled with secondary minerals like quartz, calcite, or zeolites); Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) groundmass with crystalline secondary mineral fillings.
- Hardness
- 6 on Mohs scale
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6 on Mohs scale; Color: Reddish-brown matrix with lighter spots (amygdules filled with secondary minerals like quartz, calcite, or zeolites); Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) groundmass with crystalline secondary mineral fillings.
Formation & geological history
Formed from volcanic lava flows where gas bubbles (vesicles) are trapped as the lava cools. Later, mineral-rich groundwater percolates through the rock, depositing secondary minerals into the empty vesicles to form amygdules.
Uses & applications
Used primarily in geological education, as a decorative stone, and occasionally in road construction or landscaping.
Geological facts
The distinct spots on this rock are known as 'amygdules' or 'almonds' derived from the Greek word for almond. When weathered out, these mineral fillings can sometimes yield agates or zeolites.
Field identification & locations
Can be identified in the field by its dark or reddish volcanic groundmass speckled with distinct rounded or oval mineral-filled cavities. Commonly found in regions with historic volcanic activity, such as the Lake Superior region and basalt plateaus worldwide.
