igneous

Rhyolite (Porphyritic)

Rhyolite (Felsic Extrusive Igneous Rock)

Hardness: 6-7 on Mohs scale; Color: typically light gray, tan, or pinkish; Luster: dull to earthy groundmass with vitreous (glassy) phenocrysts; Structure: Aphanitic to porphyritic with visible crystals of quartz or feldspar; Specific Gravity: 2.4-2.6

Hardness: 6-7 on Mohs scale
Color: typically light gray, tan, or pinkish
Luster: dull to earthy groundmass with vitreous (glassy) phenocrysts

Identified February 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of high-silica (felsic) lava erupted from volcanoes. Can range from Precambrian to Holocene in age. Often associated with continental volcanic arcs and caldera-forming eruptions.

Uses & applications

Commonly used as crushed stone for road construction and concrete aggregate. Historically used for stone tools (knapping) due to its high silica content and conchoidal fracture. Some banded varieties are polished for lapidary use.

Geological facts

Rhyolite is the volcanic equivalent of granite; they share the same chemical composition but differ in grain size. Extremely explosive volcanic eruptions, like those at Yellowstone, are typically fueled by rhyolitic magma due to its high viscosity.

Field identification & locations

Identify in the field by its light color, fine-grained 'stony' texture, and the presence of small, glassy quartz or feldspar crystals (phenocrysts). Found in volcanic regions like the American Southwest, Iceland, and New Zealand. Collect specimens with unique banding or large crystals.

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