igneous

Red Rhyolite Porphyry

Rhyolite (Porphyritic texture)

Hardness: 6-7 on Mohs scale; Color: Brick red to reddish-brown groundmass with white or light-grey phenocrysts; Luster: Dull (stony) to vitreous in phenocrysts; Crystal Structure: Aphanitic matrix with visible crystals (porphyritic); Cleavage: None/Irregular; Specific Gravity: 2.4-2.6.

Hardness: 6-7 on Mohs scale
Luster: Dull (stony) to vitreous in phenocrysts

Identified March 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed through rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. The larger white crystals (phenocrysts) formed slowly at depth before the remaining magma erupted and cooled quickly. These are often associated with volcanic arcs and can range from recent to hundreds of millions of years old.

Uses & applications

Used primarily as decorative crushed stone, road aggregate, or in landscaping. Polished specimens are sometimes used as pocket stones or in jewelry (cabochons) due to the attractive contrast between the red base and white spots.

Geological facts

Rhyolite has the same chemical composition as granite but is extrusive rather than intrusive. The red color is typically caused by the oxidation of iron-rich minerals (hematite) within the volcanic glass or quartz matrix.

Field identification & locations

Identify in the field by its hardness (will scratch glass), its red earthy matrix, and the presence of small, distinct lighter spots (usually feldspar or quartz). It is commonly found in volcanic regions such as the American Southwest, Lake Superior basin, and parts of the UK.

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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)

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Apatite (Group), Ca5(PO4)3(F,Cl,OH)

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Magnetite

Magnetite (Fe3O4)

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