igneous

Porphyritic Andesite (often called Chinese Writing Stone)

Porphyritic Basaltic Andesite with Feldspar Phenocrysts

Hardness: 6-7 (Mohs scale), Color: Dark grey to black groundmass with white lath-shaped crystals, Luster: Dull to sub-vitreous, Crystal structure: Porphyritic (large crystals in a fine-grained matrix), Cleavage: Good in crystals, poor in matrix, Specific gravity: 2.5 - 2.8

Identified March 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed from volcanic activity where magma cooled in two stages: slowly underground (growing large crystals) and then rapidly on or near the surface (forming the fine-grained matrix). These rocks are typically from the Mesozoic era.

Uses & applications

Used primarily for lapidary work, jewelry (cabs/beads), and ornamental carvings. Historically used for acupuncture stones in some Asian cultures. Industrially used as crushed stone or road base.

Geological facts

The name 'Chinese Writing Stone' comes from the way the white elongated feldspar crystals intersect, resembling calligraphic strokes. It is not actually jade, despite sometimes being marketed as 'Basalt Jade'.

Field identification & locations

Identify in the field by the distinct needle-like or rice-grain-shaped white crystals scattered randomly in a dark dense rock. Common in California (Auburn/Sierras), Canada, and Asia. Collectors look for sharp contrast between the 'ink' (matrix) and 'pen' (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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral