igneous
Basalt (Vesicular or Porphyritic River Pebble)
Mafic Volcanic Rock (Aphanitic to Porphyritic Basalt)
Hardness: 6-7 on Mohs scale; Color: Dark olive green to dark grayish-black with lighter mineral speckles and a quartz/feldspar vein; Luster: Dull to earthy (water-worn matte finish); Crystal Structure: Aphanitic (fine-grained matrix) with small plagioclase/olivine phenocrysts or mineral-filled vesicles;…
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to earthy (water-worn matte finish)
Identified More igneous →
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark olive green to dark grayish-black with lighter mineral speckles and a quartz/feldspar vein; Luster: Dull to earthy (water-worn matte finish); Crystal Structure: Aphanitic (fine-grained matrix) with small plagioclase/olivine phenocrysts or mineral-filled vesicles; Cleavage: None (conchoidal to uneven fracture).
Formation & geological history
Formed by the rapid cooling of magnesium-rich and iron-rich lava at or near the surface of the Earth. Subsequent stream, river, or glacial action rounded and smoothed the rock into a pebble, exposing a thin quartz/calcite vein filling a fracture.
Uses & applications
Commonly used in construction aggregate, road base, river rock landscaping, and hot stone massage therapy. As a collector pebble, it is valued for decorative aquariums or rock tumbling.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the oceanic crust. Rocks like this often contain tiny quartz or epidote veins that formed when mineral-rich hydrothermal fluids filled fractures in the cooling lava flow.
Field identification & locations
Identified in the field by its dark greenish-gray color, heavy fine-grained dense mass, rounded stream-worn texture, and a distinct pale quartz vein cutting across the surface. Commonly found along riverbeds, beaches, and glacial till deposits worldwide.
