igneous
Porphyritic Basalt
Porphyritic Basalt (containing Plagioclase phenocrysts)
Hardness: 6-7 on Mohs scale; Color: Dark grey, greenish-black, or brown groundmass with white rectangular spots; Luster: Dull to earthy (groundmass), vitreous (crystals); Structure: Porphyritic (large crystals in fine matrix); Specific Gravity: 2.8 - 3.0
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to earthy (groundmass), vitreous (crystals)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey, greenish-black, or brown groundmass with white rectangular spots; Luster: Dull to earthy (groundmass), vitreous (crystals); Structure: Porphyritic (large crystals in fine matrix); Specific Gravity: 2.8 - 3.0
Formation & geological history
Formed from the cooling of mafic lava. The large white phenocrysts (plagioclase feldspar) began growing slowly deep underground before the remaining lava erupted and cooled rapidly on the surface, creating the fine-grained matrix.
Uses & applications
Used primarily in construction as crushed stone, road base, and concrete aggregate. Select specimens are used as decorative garden stones or for lapidary tumbling.
Geological facts
The white spots are called 'phenocrysts'. When they are particularly abundant or star-shaped, these rocks are sometimes colloquially referred to as 'Chinese Writing Stone' or 'Flowerstone' in the lapidary trade.
Field identification & locations
Identify by looking for distinct, often rectangular, white crystals set within a much darker, fine-grained rock. Commonly found in volcanic regions, riverbeds near basaltic flows, and glacial till.
