igneous
Amygdaloidal Basalt (or Orbicular/Vesicular Basalt)
Basalt with quartz/agate amygdules
Hardness: 6-7 (for quartz fillings), 5-6 (for basalt host); Color: Dark brown to black matrix with lighter white, gray, or yellowish rounded spots (amygdules); Luster: Dull to vitreous in the cavities; Crystal structure: Microcrystalline to cryptocrystalline in the amygdules, aphanitic in the groundmass;…
- Hardness
- 6-7 (for quartz fillings), 5-6 (for basalt host)
- Luster
- Dull to vitreous in the cavities
Identified More igneous →
Explore Amygdaloidal Basalt (or Orbicular/Vesicular Basalt) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 (for quartz fillings), 5-6 (for basalt host); Color: Dark brown to black matrix with lighter white, gray, or yellowish rounded spots (amygdules); Luster: Dull to vitreous in the cavities; Crystal structure: Microcrystalline to cryptocrystalline in the amygdules, aphanitic in the groundmass; Cleavage: None; Specific gravity: 2.8-3.0.
Formation & geological history
Formed from mafic lava flows where gas bubbles created vesicles that were later filled by secondary mineral solutions (like silica or zeolites) precipitating out of groundwater, often during the Keweenawan or other volcanic epochs.
Uses & applications
Collected by rockhounds, lapidary cabochons, and educational geological specimens.
Geological facts
The spots are known as amygdules or amygdaloids. When the soft mineral fillings weather out faster than the hard basalt matrix, it leaves behind distinctive pitted holes, sometimes referred to as 'puddingstone' or 'button basalt' depending on the exact regional nomenclature.
Field identification & locations
Identified by its dark volcanic matrix dotted with distinct round or oval filled vesicles (or hollow pits where they weathered out). Commonly found in ancient volcanic regions such as the Lake Superior basin, Pacific Northwest, and various basalt traps worldwide.
