igneous

Andesite

Andesite (intermediate volcanic rock)

Hardness: 6-7 on Mohs scale; Color: Gray to grayish-green; Luster: Dull or earthy (aphanitic texture); Crystal Structure: Fine-grained matrix with occasional larger phenocrysts; Specific Gravity: 2.4 - 2.8.

Hardness: 6-7 on Mohs scale
Color: Gray to grayish-green
Luster: Dull or earthy (aphanitic texture)

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of intermediate lava at or near the earth's surface, typically at convergent plate boundaries (subduction zones). It consists mainly of plagioclase feldspar, pyroxene, and hornblende.

Uses & applications

Used primarily in the construction industry as crushed stone for road base, aggregate in concrete, and as a fill material. Quality specimens are sometimes used as decorative stone in landscaping.

Geological facts

Andesite is the second most common volcanic rock in the Earth's crust after basalt. It is named after the Andes Mountains, where it is found in great abundance due to the ongoing subduction of the Nazca plate.

Field identification & locations

Identify in the field by its characteristic gray color and fine-grained 'salt and pepper' appearance. It is typically lighter in color than basalt but darker than rhyolite. Found globally in volcanic arcs like the Cascades, 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