igneous

Porphyritic Basalt (Drill Core)

Porphyritic Basalt with Plagioclase Phenocrysts

Hardness: 6 (matrix) to 6.5 (crystals); Color: Dark grey to black groundmass with light green/white phenocrysts; Luster: Glassy to dull; Crystal structure: Porphyritic (large crystals in fine-grained matrix); Specific gravity: 2.8-3.0

Hardness: 6 (matrix) to 6
Luster: Glassy to dull

Identified May 27, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava at or near the Earth's surface, where large crystals (phenocrysts) began forming at depth before a final eruption quenched the remaining melt into a fine-grained matrix.

Uses & applications

Used in geological surveying to analyze subsurface volcanic structures, petroleum exploration, mineral prospecting, and as high-quality construction aggregate.

Geological facts

This specimen is a 'drill core,' a cylindrical section of rock obtained by a specialized drill. The large green-tinted crystals are likely plagioclase feldspar that achieved their size through slow cooling before the rock was fully extruded.

Field identification & locations

Identify by its cylindrical shape (from drilling), the contrast between large rectangular crystals and the dark matrix. Common in volcanic regions such as the Pacific Northwest or oceanic islands.

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