igneous
Porphyritic Basalt (Chinese Writing Stone)
Porphyritic Basaltic Andesite
Hardness: 5-6 (Mohs scale), Color: Dark grey to black groundmass with white or cream-colored crystals, Luster: Dull to sub-vitreous, Crystal structure: Porphyritic (large crystals in fine-grained matrix), Cleavage: None/Irregular
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Physical properties
Hardness: 5-6 (Mohs scale), Color: Dark grey to black groundmass with white or cream-colored crystals, Luster: Dull to sub-vitreous, Crystal structure: Porphyritic (large crystals in fine-grained matrix), Cleavage: None/Irregular
Formation & geological history
Formed from the cooling of magma in two distinct stages: a slow cooling phase underground creating large plagioclase crystals, followed by a rapid eruption and cooling that formed the fine-grained dark matrix.
Uses & applications
Used primarily as a decorative stone, in lapidary work for making cabochons, beads, and as a unique collector's specimen.
Geological facts
The nickname 'Chinese Writing Stone' comes from the way the white feldspar crystals are oriented, which resembles characters of Chinese calligraphy.
Field identification & locations
Identify by looking for distinct, needle-like or lath-shaped white crystals (phenocrysts) scattered randomly through a dark, dense volcanic rock. Commonly found in California (USA) and portions of Asia.
