igneous
Basalt Porphyry
Porphyritic Basalt
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with lighter white/grey crystals, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic matrix with phenocrysts (porphyritic), Cleavage: Indistinct, Specific Gravity: 2.8-3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with lighter white/grey crystals, Luster: Dull to sub-metallic, Crystal Structure: Aphanitic matrix with phenocrysts (porphyritic), Cleavage: Indistinct, Specific Gravity: 2.8-3.0
Formation & geological history
Formed from volcanic lava that cooled in two stages: initially cooling slowly underground to form large crystals (phenocrysts), followed by a rapid eruption and quick cooling of the remaining melt to form the fine-grained matrix.
Uses & applications
Used primarily in construction as crushed stone or aggregate for roads and concrete. Smooth river-worn specimens are popular in landscaping and decorative gardening.
Geological facts
This specific piece is a water-rounded pebble, likely smoothed by river or beach action. Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.
Field identification & locations
Identify in the field by its dark, fine-grained mass containing visible, distinct light-colored crystals (often plagioclase feldspar). Commonly found in volcanic regions or in glacial till/riverbeds moved from volcanic sources.
