igneous
Porphyritic basalt or andesite
Porphyritic volcanic rock; typically plagioclase phenocrysts in a fine mafic/intermediate groundmass
AI-generated identification · may be incorrectDark gray, fine-grained matrix with abundant pale angular crystals, likely plagioclase; generally vitreous to dull luster, hardness about 5–6, no obvious cleavage, and specific gravity roughly 2.7–3.0. The rounded shape appears water-worn or tumbled.
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Physical properties
Dark gray, fine-grained matrix with abundant pale angular crystals, likely plagioclase; generally vitreous to dull luster, hardness about 5–6, no obvious cleavage, and specific gravity roughly 2.7–3.0. The rounded shape appears water-worn or tumbled.
Formation & geological history
Formed when lava cooled relatively quickly at or near Earth’s surface, preserving larger crystals formed earlier in magma; commonly Cenozoic or older, depending on locality. Exact composition and age require testing.
Uses & applications
Used as aggregate, road base, riprap, and building stone; attractive pieces may be collected, but this small specimen has little commercial value.
Geological facts
Its conspicuous large crystals in a fine matrix define a porphyritic texture, indicating two-stage cooling. The pale grains could also include altered feldspar or other volcanic minerals.
Field identification & locations
Identify by its dark volcanic matrix, scattered light feldspar crystals, dense weight, and lack of layering or visible sedimentary cement. Common in volcanic regions worldwide, including the Andes, Cascades, Hawaii, Iceland, and continental rift zones.
