igneous
Porphyritic Basalt
Porphyritic Aphanitic Basalt with Plagioclase Phenocrysts
Hardness: 6-7 (Mohs); Color: Dark grey to reddish-brown groundmass with light-colored lath-like crystals; Luster: Dull to sub-metallic (weathered); Crystal structure: Porphyritic (two distinct grain sizes); Cleavage: Distinct in crystals, none in matrix.
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to sub-metallic (weathered)
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to reddish-brown groundmass with light-colored lath-like crystals; Luster: Dull to sub-metallic (weathered); Crystal structure: Porphyritic (two distinct grain sizes); Cleavage: Distinct in crystals, none in matrix.
Formation & geological history
Formed from volcanic lava cooling in two stages: first, slow cooling underground creates the larger crystals (phenocrysts), followed by rapid cooling at or near the surface which solidifies the fine-grained matrix.
Uses & applications
Primarily used as construction aggregate, road base, and occasionally as decorative landscaping stone or for lapidary 'flower stone' carving if the pattern is exceptional.
Geological facts
The distinct needle-like or lath-like white crystals are typically Plagioclase feldspar. When this specific pattern is found in certain regions of Japan or the Pacific Northwest, it is often referred to as 'Chinese Writing Stone' or 'Chrysanthemum Stone' depending on the exact crystal arrangement.
Field identification & locations
Identified in the field by the 'chocolate chip cookie' texture where large, visible crystals are embedded in a much finer, dark-colored rock. Common in volcanic terrains and riverbeds near oceanic-continental plate boundaries.
