igneous

Porphyritic Rhyolite (Red Porphyry)

Porphyritic Rhyolite (Felsic volcanic rock)

Hardness: 6-7 on Mohs scale; Color: Reddish-brown to maroon matrix with white or cream-colored phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained groundmass with larger visible crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4-2.6

Hardness: 6-7 on Mohs scale
Luster: Dull to sub-vitreous

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich lava at or near the Earth's surface, where larger crystals (phenocrysts) began forming at depth before a secondary eruption caused a fine-grained matrix to solidify. Often dates from the Proterozoic to more recent volcanic eras.

Uses & applications

Used primarily as decorative stone, construction aggregate, and for lapidary work such as cabochons or polished river rocks. Historically used by Romans as 'Imperial Porphyry' for statues and pillars.

Geological facts

The term 'Porphyry' comes from the Ancient Greek word for purple. This rock's unique 'chocolate chip' appearance is caused by a two-stage cooling process of the magma.

Field identification & locations

Identify by looking for distinct, rectangular or blocky white feldspar crystals embedded in a hard, fine-grained reddish matrix. Commonly found in glaciated regions (as eratics), volcanic fields, and riverbeds in areas like the American Southwest, Egypt, or Northern Europe.

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