igneous
Porphyritic Basalt (with Olivine or Quartz phenocryst)
Porphyritic Basalt (Mafic Extrusive)
Hardness: 6-7 Mohs; Color: Dark grey to black matrix with a greenish-white inclusion; Luster: Dull to earthy; Texture: Aphanitic to porphyritic; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 Mohs
- Color
- Dark grey to black matrix with a greenish-white inclusion
- Luster
- Dull to earthy
Identified More igneous →
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Physical properties
Hardness: 6-7 Mohs; Color: Dark grey to black matrix with a greenish-white inclusion; Luster: Dull to earthy; Texture: Aphanitic to porphyritic; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The large inclusion (phenocryst) formed earlier in the cooling process deeper underground before being carried to the surface.
Uses & applications
Commonly used as crushed stone for construction, road base, and railroad ballast. This specific small, water-worn pebble is primarily a collector's curiosity or a decorative river stone.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The large greenish inclusion visible in this specimen is likely a phenocryst of olivine or a quartz amygdule/vein fragment.
Field identification & locations
Identify by its dark, dense matrix and visible large mineral crystals (phenocrysts). Commonly found in volcanic regions, riverbeds, and coastal areas near tectonic boundaries.
