igneous

Porphyry (specifically Porphyritic Basalt or Andesite)

Porphyritic extrusive igneous rock (Mg,Fe)2SiO4 (Olivine inclusions/Plagioclase Feldspar)

Hardness: 6-7 (Mohs scale); Color: Dark gray/greenish groundmass with white or light-cream phenocrysts; Luster: Dull/Sub-vitreous; Crystal Structure: Porphyritic (large crystals in a fine-grained matrix); Specific Gravity: 2.7–3.3

Hardness: 6-7 (Mohs scale)
Luster: Dull/Sub-vitreous

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed through a two-stage cooling process of magma: slow cooling at depth allowing large crystals (phenocrysts) to grow, followed by rapid eruption and quick cooling of the remaining melt (groundmass). Commonly associated with volcanic arcs and tectonic boundaries.

Uses & applications

Used primarily as decorative stone, crushed stone for road construction, and historical architectural sculpture. In the ancient world (Egypt/Rome), purple varieties were highly prized symbols of status.

Geological facts

The word 'porphyry' comes from the Ancient Greek word for 'purple.' Imperial Porphyry from Egypt was so restricted that it was only used by Roman Emperors for sarcophagi and palace floors.

Field identification & locations

Identify in the field by the distinct 'polka-dot' appearance: large, well-defined angular crystals embedded in a solid, fine-grained rock. Commonly found in areas of historical volcanic activity such as the Pacific Northwest, Greece, and Egypt.

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral