igneous
Chinese Writing Stone
Porphyritic Basalt or Porphyritic Andesite
Hardness: 5-6 (Mohs scale), Color: Dark grey to black matrix with white or cream-colored inclusions, Luster: Vitreous to dull, Crystal Structure: Porphyritic (long lath-like crystals in a fine-grained groundmass), Cleavage: Perfect in two directions for the feldspar crystals.
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Physical properties
Hardness: 5-6 (Mohs scale), Color: Dark grey to black matrix with white or cream-colored inclusions, Luster: Vitreous to dull, Crystal Structure: Porphyritic (long lath-like crystals in a fine-grained groundmass), Cleavage: Perfect in two directions for the feldspar crystals.
Formation & geological history
Formed through volcanic processes where magma cools in two stages: first, large crystals grow slowly at depth; then, the surrounding magma erupts and cools quickly to form the dark matrix. These are typically from the Mesozoic era.
Uses & applications
Primarily used for lapidary work, including cabochons, beads, decorative carvings, and as a collector's specimen due to its unique aesthetic.
Geological facts
The name comes from the white feldspar (orthoclase or plagioclase) crystals that look like ancient Chinese calligraphy characters against the dark background. It is also sometimes called 'Porphyry' or 'Cinnabar Stone' (though it contains no cinnabar).
Field identification & locations
Identify by the high-contrast white elongated crystals embedded in a dense dark matrix. Commonly found in the Yangtze River region of China and certain areas of California, USA.
