igneous
Amygdaloidal Basalt
Extrusive mafic igneous rock with amygdules
Hardness of 6 on the Mohs scale for the base rock, color typically dark grey to green-grey with lighter weathered spots or filled vesicles (amygdules), luster is dull to sub-vitreous, fine-grained aphanitic crystal structure with distinct rounded secondary mineral fillings.
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Physical properties
Hardness of 6 on the Mohs scale for the base rock, color typically dark grey to green-grey with lighter weathered spots or filled vesicles (amygdules), luster is dull to sub-vitreous, fine-grained aphanitic crystal structure with distinct rounded secondary mineral fillings.
Formation & geological history
Formed from cooling basaltic lava flows where gas bubbles created vesicles that were later filled by hydrothermal mineral solutions, often dating from various geological eras including Precambrian to recent.
Uses & applications
Used primarily in construction, aggregate, road metal, and sometimes as ornamental stone when highly polished or featuring attractive mineral infills.
Geological facts
The spots visible on this rounded weathered specimen are weathered amygdules, which are cavities originally formed by gas bubbles in molten lava and subsequently filled with secondary minerals like quartz, calcite, or zeolites.
Field identification & locations
Easily identified by its dark, heavy, fine-grained volcanic matrix marked by distinctive rounded spots or hollows on weathered surfaces. Found worldwide in regions with historic volcanic activity, such as the Keweenaw Peninsula of Michigan or the Columbia River Basalts.
