igneous

Porphyritic Andesite

Andesite (Porphyry)

Hardness: 6-7 Mohs; Color: Brownish-purple to dark gray matrix with white/light gray phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None (conchoidal fracture common in matrix).

Hardness: 6-7 Mohs
Luster: Dull to sub-vitreous

Identified March 24, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of intermediate lava near or at the Earth's surface, where larger crystals (phenocrysts like plagioclase) formed early in a magma chamber before the remaining liquid solidified quickly. Most common in volcanic arcs and subduction zones.

Uses & applications

Commonly used as crushed stone for road construction, aggregate in concrete, and architectural decorative stone. In some regions, it is used for building tiles and monuments due to its durability.

Geological facts

Andesite is the second most common volcanic rock in Earth's crust after basalt. It is named after the Andes Mountains, where it is found in great abundance along the subduction zone.

Field identification & locations

Identify by looking for 'polka-dot' crystals set in a fine, dark, earthy-toned background. Found globally in volcanic regions like the American Northwest (Cascades), the Andes, and Japan. Collectors look for high contrast between crystals and matrix.

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