igneous
Porphyritic Basalt (Chinese Writing Stone)
Porphyritic Basalt or Porphyritic Andesite
Hardness: 6-7 (Mohs scale); Color: Dark grey to black groundmass with creamy white to tan phenocrysts; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic matrix with lath-like Plagioclase Feldspar crystals; Specific Gravity: 2.7-3.0
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to black groundmass with creamy white to tan phenocrysts; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic matrix with lath-like Plagioclase Feldspar crystals; Specific Gravity: 2.7-3.0
Formation & geological history
Formed through a two-stage cooling process of volcanic magma. Large plagioclase crystals (phenocrysts) grew slowly deep underground before the remaining magma was erupted and cooled rapidly, creating the fine-grained dark matrix.
Uses & applications
Primarily used for ornamental purposes, lapidary work (cabochons, beads), decorative garden stones, and as a collector's specimen due to its unique pattern.
Geological facts
Commonly known as 'Chinese Writing Stone' or 'Chinese Writing Rock' because the random orientation of the light-colored crystals against the dark background resembles calligraphy characters.
Field identification & locations
Identify in the field by the distinct light-colored needle or lath-shaped crystals embedded in a dark, fine-grained volcanic rock. Often found as river-worn cobbles in areas of ancient volcanic activity, such as parts of California, Nevada, and China.
