igneous
Vesicular basalt with a pale mineral infill
Basalt, likely amygdaloidal basalt; infill possibly calcite or zeolite
AI-generated identification · may be incorrectDark fine-grained volcanic rock with abundant vesicles (gas bubbles), reddish-brown weathering or surface coating, and a white botryoidal to granular filling. Basalt is typically Mohs 5–6, dull to slightly glassy, with no obvious cleavage; calcite infill is softer (Mohs 3) and may fizz in dilute acid.
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Physical properties
Dark fine-grained volcanic rock with abundant vesicles (gas bubbles), reddish-brown weathering or surface coating, and a white botryoidal to granular filling. Basalt is typically Mohs 5–6, dull to slightly glassy, with no obvious cleavage; calcite infill is softer (Mohs 3) and may fizz in dilute acid.
Formation & geological history
Formed when basaltic lava rapidly cooled at or near the surface, trapping gas bubbles; later groundwater deposited carbonate or zeolite minerals in the cavities. Basalt ranges from recent lava flows to Precambrian volcanic formations.
Uses & applications
Used as crushed aggregate, roadstone, riprap, building stone, and decorative material; attractive amygdules are collected as specimens. Value is generally modest unless the cavity mineral is unusually attractive.
Geological facts
The cavities are called vesicles; once mineral-filled, they are called amygdules, producing amygdaloidal basalt. The reddish color commonly reflects iron oxidation (hematite or limonite).
Field identification & locations
Identify by its dense dark volcanic matrix, irregular holes, and possible magnetic response from magnetite. Common worldwide in volcanic regions, including Iceland, India, Hawaii, the Pacific Northwest, and oceanic crust.
