igneous

Porphyritic Andesite

Porphyritic Andesite (Intermediate Volcanic Rock)

Hardness: 6-7 on Mohs scale; Color: Dark gray to greenish-gray 'groundmass' with white to light-colored rectangular crystals; Luster: Dull to sub-vitreous; Crystal Structure: Porphyritic (large crystals in fine-grained matrix); Cleavage: Good in two directions for the feldspar phenocrysts; Specific Gravity: 2.4-2.8

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

Identified March 22, 2026More igneous →

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Physical properties

Formation & geological history

Formed through a two-stage cooling process of intermediate magma. Initial slow cooling deep underground allows large Plagioclase feldspar crystals (phenocrysts) to grow, followed by rapid cooling during a volcanic eruption which freezes the remaining magma into a fine-grained 'groundmass'. Commonly found in volcanic arcs above subduction zones.

Uses & applications

Used primarily in construction as crushed stone for road beds, railway ballast, and occasionally as decorative facing stone or garden ornaments due to its unique 'starry' or 'snow-on-rock' appearance.

Geological facts

This specific texture is often colloquially called 'Chinese Writing Stone' or 'Chrysanthemum Stone' (though true Chrysanthemum stone is different). It captures a snapshot in time of a volcano's plumbing system right before an eruption.

Field identification & locations

Identify in the field by looking for distinct, lath-shaped white crystals that look like scattered grains of rice or matchsticks embedded in a dark, hard rock. Common in the Andes Mountains, Cascades (USA), and Japan. Collectors should look for high contrast between the white crystals and dark background.

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