igneous

Porphyritic Andesite

Andesite (with white Plagioclase phenocrysts)

Hardness: 6-7 (Mohs scale), Color: Dark grey to charcoal matrix with white speckles, Luster: Dull/earthy matrix with vitreous minerals, Structure: Porphyritic (large crystals in fine-grained groundmass), Cleavage: Perfect on plagioclase crystals.

Identified May 28, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the cooling of intermediate silicate lavas at volcanic arcs above subduction zones. The larger crystals (phenocrysts) grew slowly at depth before the final eruption quenched the remaining magma.

Uses & applications

Primarily used in construction for crushed stone, road base, and occasionally as decorative stone or architectural facing.

Geological facts

Andesite is the second most common volcanic rock in the Earth's crust after basalt and takes its name from the Andes Mountains where it is abundant.

Field identification & locations

Identify by looking for 'salt and pepper' appearance with distinct rectangular white crystals set in a fine, dark background. Common in the Pacific Northwest, the Andes, and Japan.

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

Dalmatian Jasper

Aplite with Arfvedsonite inclusions

igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral