igneous

Porphyry

Porphyritic Basalt or Porphyritic Andesite

Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-black groundmass with light cream or white phenocrysts; Luster: Dull to earthy; Structure: Porphyritic (large crystals in fine-grained matrix); Specific Gravity: 2.5-3.0.

Hardness: 6-7 (Mohs scale)
Luster: Dull to earthy

Identified May 16, 2026More igneous →

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Physical properties

Formation & geological history

Formed when magma cools in two stages: first, large crystals (phenocrysts) grow slowly underground, then the remaining melt erupts or moves closer to the surface, cooling rapidly into a fine-grained matrix. Often associated with volcanic arcs and tectonic plate boundaries.

Uses & applications

Historically used for sculpture and monuments (notably Imperial Porphyry in Rome); currently used for decorative stone, road aggregate, and as building cladding.

Geological facts

The term 'Porphyry' comes from the Ancient Greek word for purple. Ancient Egyptians and Romans prized 'Imperial Porphyry' for its deep purple color, which became a symbol of royalty and power.

Field identification & locations

Identify by the 'chocolate chip cookie' appearance—distinct large, lighter crystals (usually feldspar) embedded in a significantly finer, darker rock base. Common in glaciated regions as erratic stones or in volcanic mountain ranges.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

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