igneous
Amygdaloidal Basalt
Basalt (with filled vesicles/amygdules)
Hardness: 5.5-6 (matrix); Color: Dark brown to black matrix with whitish/light-colored secondary mineral fillings (amygdules); Luster: Dull to earthy matrix with vitreous inclusions; Texture: Aphanitic to vesicular/amygdaloidal; Cleavage: Poor
- Hardness
- 5
- Luster
- Dull to earthy matrix with vitreous inclusions
Identified More igneous →
Explore Amygdaloidal Basalt in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 5.5-6 (matrix); Color: Dark brown to black matrix with whitish/light-colored secondary mineral fillings (amygdules); Luster: Dull to earthy matrix with vitreous inclusions; Texture: Aphanitic to vesicular/amygdaloidal; Cleavage: Poor
Formation & geological history
Formed when gas bubbles (vesicles) trapped in cooling basaltic lava flows were later filled with secondary minerals such as zeolites, quartz, calcite, or chalcedony deposited by percolating hydrothermal fluids.
Uses & applications
Mainly collected by rockhounds and educators as a classic geological specimen illustrating volcanic gas retention and secondary mineralization; also used in crushed stone aggregate.
Geological facts
The word 'amygdaloidal' comes from the Greek word for 'almond', referring to the almond-shaped fillings in the gas cavities. In places like the Lake Superior region, copper or agate often fills these amygdules.
Field identification & locations
Identified by a fine-grained, dark volcanic rock matrix studded with rounded or oval mineral-filled cavities. Common in ancient continental flood basalt provinces and oceanic volcanic belts.
