igneous
Amygdaloidal Basalt
Amygdaloidal Basalt (Mafic Volcanic Rock)
Hardness: 5-6 (base) / 6-7 (mineral infill); Color: Dark grey or greenish-grey groundmass with white/tan almond-shaped inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Crystal structure: Aphanitic (fine-grained) with secondary mineral infilling.
- Hardness
- 5-6 (base) / 6-7 (mineral infill)
- Luster
- Dull/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: 5-6 (base) / 6-7 (mineral infill); Color: Dark grey or greenish-grey groundmass with white/tan almond-shaped inclusions; Luster: Dull/earthy matrix with vitreous inclusions; Crystal structure: Aphanitic (fine-grained) with secondary mineral infilling.
Formation & geological history
Formed from the cooling of gas-rich basaltic lava. As the lava cools, gas bubbles leave behind vesicles. Over geological time, groundwater carries dissolved minerals like quartz, calcite, or zeolites through the rock, precipitating them inside these holes (amygdules). Common in Cenozoic and Mesozoic volcanic flows.
Uses & applications
Primarily used as a decorative specimen for collectors or as landscaping stone. Historically, the groundmass of basalt is used in road construction and as an industrial abrasive.
Geological facts
The term 'amygdaloidal' comes from the Latin 'amygdala', meaning almond, referring to the characteristic shape of the mineral-filled vesicles. This specific specimen appears to be water-worn, likely found in a riverbed or on a beach.
Field identification & locations
Identify in the field by looking for a dark, heavy volcanic rock featuring distinct light-colored 'dots' or 'almonds' of mineral. Found in volcanic regions like the Lake Superior basin, the Pacific Northwest, and Iceland.
