igneous
Orbicular basalt (likely amygdaloidal basalt)
Basalt, primarily plagioclase feldspar and pyroxene with vesicles/amygdules
AI-generated identification · may be incorrectDark gray-black, fine-grained, with abundant pale rounded cavities or mineral-filled vesicles; typically vitreous to dull luster, massive texture, hardness about 6, and no obvious cleavage. The glossy surface indicates tumbling or polishing.
Identified More igneous →
Explore Orbicular basalt (likely amygdaloidal basalt) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Dark gray-black, fine-grained, with abundant pale rounded cavities or mineral-filled vesicles; typically vitreous to dull luster, massive texture, hardness about 6, and no obvious cleavage. The glossy surface indicates tumbling or polishing.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near Earth’s surface, trapping gas bubbles later filled by pale minerals such as calcite, zeolites, or chalcedony. Basalt is generally Cenozoic to Precambrian depending on locality; the specimen’s exact age cannot be determined visually.
Uses & applications
Used as aggregate, road stone, riprap, landscaping stone, and decorative polished material; attractive pieces are collected as lapidary specimens.
Geological facts
The pale spots are likely amygdules rather than true mineral crystals, formed when secondary minerals filled vesicles. Similar material is commonly sold as polished basalt or vesicular basalt.
Field identification & locations
Identify by its black fine-grained volcanic groundmass, rounded pale fillings, high density, and lack of layering. Common sources include volcanic regions worldwide, especially India, Mexico, Oregon, and the Pacific Northwest.
