igneous
Amygdaloidal Basalt
Basalt with Secondary Mineral Infillings
Hardness: 5-7 on Mohs scale; Color: Dark grey to reddish-brown matrix with white or light-colored circular spots; Luster: Dull (matrix) to vitreous (fillings); Crystal structure: Aphanitic (fine-grained) with larger mineral nodes.
- Hardness
- 5-7 on Mohs scale
- Luster
- Dull (matrix) to vitreous (fillings)
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Physical properties
Hardness: 5-7 on Mohs scale; Color: Dark grey to reddish-brown matrix with white or light-colored circular spots; Luster: Dull (matrix) to vitreous (fillings); Crystal structure: Aphanitic (fine-grained) with larger mineral nodes.
Formation & geological history
Formed from volcanic lava flows where gas bubbles (vesicles) were trapped. Over geological time, groundwater carrying dissolved minerals like quartz, calcite, or zeolites percolated through the rock and filled the cavities.
Uses & applications
Commonly used in landscaping, as road aggregate, or as decorative specimens for lapidary work and collectors.
Geological facts
The word 'Amygdaloid' comes from the Greek word 'amygdale' meaning almond, referring to the almond-like shape of the mineral fillings in the rock.
Field identification & locations
Identify in the field by looking for a fine-grained, dark volcanic rock peppered with distinct 'polka dots' of lighter mineral. Commonly found in the Lake Superior region of North America and volcanic fields worldwide.
