igneous
Porphyritic Basalt
Porphyritic Mafic Volcanic Rock
Hardness: 6 (Mohs scale); Color: Dark grey to black groundmass with white/tan large crystals (phenocrysts); Luster: Dull to sub-vitreous; Structure: Fine-grained matrix with larger blocky crystals; Specific Gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black groundmass with white/tan large crystals (phenocrysts); Luster: Dull to sub-vitreous; Structure: Fine-grained matrix with larger blocky crystals; Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed via a two-stage cooling process of magma; large crystals formed slowly underground, followed by a rapid eruption that cooled the remaining liquid into a fine-grained matrix. Often originates from Proterozoic or Cenozoic volcanic activity.
Uses & applications
Used in construction as crushed stone, garden decor, and by collectors as display specimens due to the 'flower' or 'snowflake' patterns.
Geological facts
This specific look, where light crystals resemble petals against a dark background, is sometimes colloquially called 'Chinese Writing Stone' or 'Chrysanthemum Stone' (though true Chrysanthemum stone is different).
Field identification & locations
Identified by the distinct contrast between the dark fine-grained base and the large, light-colored feldspar crystals. Commonly found in areas of old volcanic activity, such as the Pacific Northwest of the USA, Canada, and parts of China.
