igneous
Porphyritic Basalt (Chinese Writing Stone)
Porphyritic Basalt with Feldspar Phenocrysts
Hardness: 6-6.5 (Mohs); Color: Dark grey to black matrix with white or cream-colored crystals; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic matrix with lath-like phenocrysts; Cleavage: None in rock, perfect in feldspar crystals; Specific Gravity: 2.8-3.0
- Hardness
- 6-6
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-6.5 (Mohs); Color: Dark grey to black matrix with white or cream-colored crystals; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic matrix with lath-like phenocrysts; Cleavage: None in rock, perfect in feldspar crystals; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from rapid cooling of volcanic lava that began cooling slowly underground. This two-stage cooling process allows large crystals (phenocrysts) to form before the remaining magma erupts and cools quickly into a fine-grained matrix. Often associated with Mesozoic to Cenozoic volcanic activity.
Uses & applications
Used primarily as decorative stone, lapidary carvings, cabochons for jewelry, and as a unique collector's specimen. In larger quantities, basalt is used as crushed stone in construction.
Geological facts
Commonly known as 'Chinese Writing Stone' because the white feldspar crystals can resemble ancient calligraphy or characters against the dark background. It is chemically very similar to regular basalt but visually distinctive due to its texture.
Field identification & locations
Identify in the field by the 'porphyritic texture'—distinct white 'laths' or rectangular shapes embedded in a very fine-grained, dark, heavy rock. Commonly found in areas with past volcanic activity like California (USA) or British Columbia (Canada).
