igneous
Amygdaloidal Basalt with Quartz Infill
Amygdaloidal Basalt (SiO2 infill)
Hardness: 6-7 (basalt/quartz); Color: Dark grey to black matrix with white/pinkish inclusions; Luster: Dull to vitreous; Structure: Vesicular/Massive; Specific Gravity: 2.8-3.0
- Hardness
- 6-7 (basalt/quartz)
- Color
- Dark grey to black matrix with white/pinkish inclusions
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (basalt/quartz); Color: Dark grey to black matrix with white/pinkish inclusions; Luster: Dull to vitreous; Structure: Vesicular/Massive; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from cooling volcanic lava containing gas bubbles (vesicles). Over millions of years, mineral-rich groundwater deposited silica (quartz/agate) into these cavities, creating 'amygdules'. Common in Precambrian volcanic belts.
Uses & applications
Primarily collected as a decorative lapidary material or geological specimen; larger deposits of basalt are used for crushed stone and construction aggregate.
Geological facts
The term 'amygdule' comes from the Greek word 'amygdale', meaning almond, referring to the almond-like shape of the mineral infills. These rocks frequently host semi-precious gemstones like agate and zeolites.
Field identification & locations
Identify by looking for a dark, fine-grained volcanic matrix surrounding lighter, rounded mineral 'spots'. Commonly found in the Lake Superior region and other ancient volcanic terrains. Use a hand lens to see the boundary between the basalt and quartz.
