igneous
Amygdaloidal Basalt
Amygdaloidal Basalt with Secondary Mineral Amygdules
Hardness: 5-6 (for the basalt matrix), up to 7 for quartz/agate amygdules. Color: Reddish-brown or dark brownish-red matrix with white, grey, or yellowish amygdules (nodules). Luster: Dull to vitreous in the cavities. Texture: Porphyritic or vesicular with secondary infilled mineral pockets.
- Luster
- Dull to vitreous in the cavities
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: 5-6 (for the basalt matrix), up to 7 for quartz/agate amygdules. Color: Reddish-brown or dark brownish-red matrix with white, grey, or yellowish amygdules (nodules). Luster: Dull to vitreous in the cavities. Texture: Porphyritic or vesicular with secondary infilled mineral pockets.
Formation & geological history
Formed from volcanic activity when gas bubbles in cooling basaltic lava create vesicles (cavities). Over time, mineral-rich hydrothermal solutions or groundwater percolate through the rock, depositing secondary minerals like quartz, chalcedony, calcite, or zeolites inside the vesicles, forming amygdules.
Uses & applications
Commonly collected by rockhounds as lapidary material for tumbling, cabochons, and decorative display specimens.
Geological facts
Amygdules often weather out of the softer host basalt more easily, which is how many individual agate or quartz nodules are found in stream beds.
Field identification & locations
Easily identified in the field by its dark or reddish volcanic matrix dotted with rounded, contrasting mineral-filled pockets. Found worldwide in regions with ancient or active basaltic volcanism, such as the Lake Superior region or the Pacific Northwest in the United States.
