igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (containing quartz/agate or zeolite fill)
Hardness 6-7 on Mohs scale depending on mineral inclusions. Color is reddish-brown matrix with white, gray, or blue-ish mineral-filled vesicles (amygdules). Earthy matrix luster with vitreous to dull 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 6-7 on Mohs scale depending on mineral inclusions. Color is reddish-brown matrix with white, gray, or blue-ish mineral-filled vesicles (amygdules). Earthy matrix luster with vitreous to dull inclusions.
Formation & geological history
Formed when gas bubbles (vesicles) trapped in cooling basaltic lava are later filled with secondary minerals such as quartz, agate, chalcedony, calcite, or zeolites precipitation from hydrothermal fluids.
Uses & applications
Mainly collected as geological specimens or lapidary material. Polished or tumbled pieces are used in jewelry and crafts.
Geological facts
This type of rock is famous in the Lake Superior region of North America (often called Thomsonite or agate-bearing basalt), where ancient continental rifting produced vast basalt flows about 1.1 billion years ago.
Field identification & locations
Identified by its distinctive reddish, brown, or dark fine-grained lava groundmass speckled with filled circular or oval bubble cavities. Frequently found along Lake Superior shores, Minnesota, Michigan, and volcanic zones worldwide.
