igneous

Porphyritic Rhyolite (Red Porp)

Porphyritic Rhyolite (Felsic volcanic rock)

Hardness: 6-7 (Mohs scale); Color: Reddish-brown to maroon matrix with white/tan light-colored phenocrysts; Luster: Dull/Earthy matrix with glassy crystals; Crystal Structure: Aphanitic (fine-grained) groundmass with visible Phenocrysts; Specific Gravity: 2.4 - 2.6.

Hardness: 6-7 (Mohs scale)
Luster: Dull/Earthy matrix with glassy crystals

Identified March 26, 2026More igneous →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. The visible crystals (phenocrysts) formed slowly underground before the magma erupted and the remaining liquid cooled quickly into the fine-grained matrix. Often associated with Proterozoic or Paleozoic volcanic activity.

Uses & applications

Primary uses include decorative stone, construction aggregate, and landscaping. Polished specimens are popular among rockhounds and can be used for lapidary work like cabochons.

Geological facts

This specific red porphyry is often found in glacial till and beach cobbles around the Great Lakes, particularly Lake Superior, where they were transported by ice sheets from Canadian volcanic ranges. It is sometimes colloquially called 'Chinese Writing Stone' if the crystals are needle-shaped, though this specimen has more blocky feldspar crystals.

Field identification & locations

Identify by looking for 'spots' or crystals of feldspar and quartz embedded in a fine-grained, brick-red or purple-brown background. Many specimens in the Great Lakes region were rounded by glacial or water action. Collectors should look for high-contrast patterns and lack of vesicles (gas bubbles).

Other igneous specimens

See all

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