igneous

Porphyritic Basalt

Porphyritic Basalt (with plagioclase phenocrysts)

Hardness: 6-7 (Mohs); Color: Dark grey to black groundmass with white/off-white phenocrysts; Luster: Dull/earthy matrix with vitreous crystals; Structure: Aphanitic groundmass with larger crystalline inclusions (porphyritic texture); Specific Gravity: 2.8–3.0.

Hardness: 6-7 (Mohs)
Luster: Dull/earthy matrix with vitreous crystals

Identified March 30, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava on the Earth's surface or shallow subsurface. The large crystals (phenocrysts) grew slowly at depth before the final eruption rapidly cooled the remaining melt into a fine-grained matrix. Found in volcanic regions ranging from the Cenozoic to the Precambrian.

Uses & applications

Used primarily in construction as crushed stone or 'road metal.' High-quality specimens are used as architectural veneers, decorative landscape stones, or educational samples for geology students due to the clear 'textbook' porphyritic texture.

Geological facts

The distinct 'salt and pepper' look is caused by plagioclase feldspar crystals. Some variations are nicknamed 'snowflake basalt' or 'starstone' depending on the density and shape of the white crystals. This rock is a primary component of the ocean floor.

Field identification & locations

Identify by looking for a very fine, dark, dense rock base embedded with clearly visible, rectangular or tabular light-colored crystals. Commonly found in the Pacific Northwest (USA), Iceland, and Deccan Traps (India). For collectors, avoid weathered pieces where crystals have fallen out.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral