igneous

Porphyritic Rhyolite (Red Rhyolite)

Rhyolite (Porphyry)

Hardness: 6-7 on Mohs scale; Color: Reddish-brown to maroon with white or pinkish phenocrysts; Luster: Dull/earthy matrix with glassy/vitreous crystals; Crystal Structure: Aphanitic matrix with larger visible crystals (porphyritic); Specific Gravity: 2.4-2.6

Hardness: 6-7 on Mohs scale
Color: Reddish-brown to maroon with white or pinkish phenocrysts
Luster: Dull/earthy matrix with glassy/vitreous crystals

Identified March 20, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. Larger crystals (phenocrysts) formed slowly underground before a volcanic eruption caused the remaining magma to cool quickly. Common in volcanic arcs and continental rift zones from various geological ages.

Uses & applications

Used primarily as decorative crushed stone in landscaping, road construction aggregate, and occasionally as structural filler. Polished specimens are used in jewelry making (cabochons) or as pocket stones for collectors.

Geological facts

Rhyolite is the volcanic equivalent of granite. The distinctive 'spots' are typically quartz or feldspar crystals that grew deep underground. Because of the high silica content, rhyolitic lava is extremely viscous and can flow like cool maple syrup.

Field identification & locations

Identify in the field by its fine-grained, reddish 'paste' matrix embedded with distinct, lighter-colored angular crystals. Often found in riverbeds or glacial tills far from the original volcanic source. Common in the Lake Superior region and Southwestern US.

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