igneous

Porphyritic Rhyolite

Hardness: 6-7 (Mohs), Color: Reddish-brown to purple with white or light gray phenocrysts, Luster: Dull or earthy matrix with vitreous crystals, Crystal Structure: Aphanitic matrix with visible phenocrysts, Cleavage: None (conchoidal fracture), Specific Gravity: 2.4-2.6

Identified May 16, 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 lava at or near the Earth's surface. The larger crystals (phenocrysts) grew slowly at depth before the final eruption. They are typically associated with continental volcanic arcs.

Uses & applications

Mainly used as decorative stone, crushed stone for construction, road ballast, and sometimes for stone tools (arrowheads) by ancient peoples due to its conchoidal fracture.

Geological facts

The name 'Rhyolite' comes from the Greek word 'rhyax' meaning stream of lava. It has the same chemical composition as granite but cooled much faster.

Field identification & locations

Identify in the field by the light-colored, glassy matrix containing distinct larger crystals (phenocrysts) that 'twinkle' in the light. Commonly found in volcanic regions like the American Southwest or the Lake Superior region.

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