igneous

Porphyritic Basalt (Chinese Writing Stone)

Porphyritic Basalt with Andalusite or Feldspar laths

Hardness: 6.0-7.0 (Mohs scale); Color: Dark grey to black matrix with white or reddish needle-like inclusions; Luster: Dull to sub-vitreous; Structure: Porphyritic (large crystals in a fine-grained matrix); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0

Hardness: 6
Luster: Dull to sub-vitreous

Identified March 24, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic lava containing slow-growing lath-like crystals. The 'needle' inclusions are typically plagioclase feldspar or andalusite. Most specimens found in the Sierra Nevada foothills date back to the Jurassic period.

Uses & applications

Primarily used as a decorative stone, for lapidary work (cabochons), jewelry, and as a unique collector's specimen. It is valued for its striking visual patterns that resemble calligraphy.

Geological facts

Also known as 'Chinese Writing Stone' or 'Porphyry.' While 'Chinese Writing Stone' is a popular trade name, the rock itself is found worldwide, with a famous deposit located in Auburn, California.

Field identification & locations

Identify in the field by the distinct light-colored, elongated lath crystals cross-cutting a dark, dense, fine-grained volcanic matrix. Commonly found in areas of past volcanic activity or in riverbeds downstream from volcanic deposits.

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Fluorapatite (Ca5(PO4)3F)

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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)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

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