igneous

Porphyry (specifically Porphyritic Rhyolite or Andesite)

Porphyritic Igneous Rock (composed of Phenocrysts in an Aphanitic groundmass)

Hardness: 6-7 (Mohs scale), Color: Dark purple to brownish-maroon with white/grey flecks, Luster: Dull to sub-vitreous, Crystal Structure: Bimodal grain size (fine matrix with larger visible crystals), Specific Gravity: 2.5-2.8

Identified March 12, 2026More igneous →

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Physical properties

Formation & geological history

Formed through a two-stage cooling process of silica-rich magma; initially cooling slowly underground to form large crystals (phenocrysts), followed by rapid cooling during or after eruption to form the fine-grained matrix. Often associated with volcanic arcs and Precambrian to Cenozoic eras.

Uses & applications

Historically used in structural masonry, monumental sculpture (famous in Ancient Rome), and modern decorative stone, road ballast, and concrete aggregate.

Geological facts

Purple porphyry was once highly prized by Roman Emperors as a symbol of power because of its royal color; the only known source in antiquity was the Imperial Porphyry Quarry in Egypt.

Field identification & locations

Identify in the field by looking for 'speckled' or 'polka-dot' textures in dense, hard volcanic rocks, often found in glacial till or riverbeds. Commonly found in areas with past volcanic activity like the Lake Superior region, the Andes, or 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