igneous
Porphyritic Basalt
Basalt (with plagioclase phenocrysts)
Hardness: 6-7 (Mohs). Color: Dark grey to black matrix with lighter cream or white inclusions. Luster: Dull to sub-vitreous. Texture: Aphanitic matrix with visible large crystals (phenocrysts).
- Hardness
- 6-7 (Mohs)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs). Color: Dark grey to black matrix with lighter cream or white inclusions. Luster: Dull to sub-vitreous. Texture: Aphanitic matrix with visible large crystals (phenocrysts).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The large crystals (phenocrysts) formed slowly underground before being erupted and encased in the fine-grained matrix.
Uses & applications
Commonly used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Distinctive specimens are often kept by amateur collectors.
Geological facts
This specific texture is often nicknamed 'Chinese Writing Stone' or 'Flower Stone' depending on the shape of the white crystals. Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.
Field identification & locations
Look for a dark, heavy, fine-grained rock with distinct, angular, light-colored crystal 'flecks'. Found globally in volcanic regions or riverbeds downstream from volcanic outcrops.
