igneous

Porphyritic Rhyolite (Red)

Rhyolite (Porphyry)

Hardness: 6-7 Mohs; Color: Reddish-brown/terracotta base with light-colored phenocrysts; Luster: Dull/Earthy matrix with Vitreous crystals; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Fracture: Subconchoidal.

Hardness: 6-7 Mohs
Color: Reddish-brown/terracotta base with light-colored phenocrysts
Luster: Dull/Earthy matrix with Vitreous crystals

Identified March 24, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of silica-rich volcanic lava at or near the Earth's surface. The larger crystals (phenocrysts) grew slowly deep underground before the lava erupted and the rest cooled quickly. Typically ranges from Cenozoic to Paleozoic in age.

Uses & applications

Used primarily as decorative aggregate, crushed stone for road construction, landscaping, and occasionally as structural stone. Dense varieties are sometimes used for polishing as decorative 'pocket stones'.

Geological facts

Rhyolite is the volcanic equivalent of granite. The red color is usually caused by the oxidation of iron within the rock during or after formation. It is extremely high in silica content (over 69%).

Field identification & locations

Identify by its 'speckled' appearance within a fine-grained, reddish-colored groundmass. It will not scratch with a pocket knife but will scratch glass. Common in volcanic regions such as the American Southwest, Rocky Mountains, and various volcanic arcs globally.

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