igneous

Andesite

Andesite (Intermediate Volcanic Rock)

Hardness: 6-7 on Mohs scale. Color: Gray to grayish-brown with fine-grained (aphanitic) texture; features small visible phenocrysts of plagioclase or hornblende. Luster: Dull to sub-vitreous. Crystal Structure: Fine-grained matrix with larger crystals (porphyritic).

Hardness: 6-7 on Mohs scale
Color: Gray to grayish-brown with fine-grained (aphanitic) texture
Luster: Dull to sub-vitreous

Identified April 1, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of intermediate-composition lava at or near the surface, typically near subduction zones or volcanic arcs. Ages vary by location, from recent Quaternary flows to ancient Paleozoic formations.

Uses & applications

Used primarily in construction for road base, aggregate, and building stone. Because of its durability, it is often used as crushed stone in paving and railroad ballast.

Geological facts

Andesite is named after the Andes Mountains where it is found in abundance. It is the volcanic equivalent of diorite and represents one of the most common rock types in volcanic island arcs.

Field identification & locations

Identify by its gray color and 'intermediate' weight (heavier than rhyolite, lighter than basalt). Common in the Pacific Northwest (USA), Japan, and South America. Collectors look for large porphyritic 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