igneous

Porphyritic Rhyolite

Rhyolite

Hardness: 6-7 (Mohs scale); Color: Pinkish-red to reddish-brown; Luster: Pearly to dull with vitreous phenocrysts; Crystal structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None (conchoidal fracture)

Hardness: 6-7 (Mohs scale)
Color: Pinkish-red to reddish-brown
Luster: Pearly to dull with vitreous phenocrysts

Identified May 23, 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. The larger phenocrysts (visible crystals) formed cooling slowly underground before the final eruption. Common in volcanic regions across various geological ages.

Uses & applications

Used as decorative stone, crushed stone for construction and road base, and occasionally in jewelry as cabochons if the pattern is particularly striking.

Geological facts

Rhyolite is the volcanic equivalent of granite. The name comes from the Greek word 'rhyax', meaning a stream of lava. Because of its high silica content, rhyolitic lava is very viscous and often erupts explosively.

Field identification & locations

Identify by the reddish pink matrix containing visible quartz or feldspar crystals (phenocrysts). Commonly found in mountainous volcanic terrains such as the American Southwest or the Cascade Range.

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