igneous
Basalt Porphyry
Porphyritic Basalt
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with black phenocrysts; Luster: Dull/Earthy (matrix) to Vitreous (crystals); Structure: Porphyritic (large crystals in fine-grained matrix); Specific Gravity: 2.8 - 3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal with black phenocrysts
- Luster
- Dull/Earthy (matrix) to Vitreous (crystals)
Identified More igneous →
Explore Basalt Porphyry in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal with black phenocrysts; Luster: Dull/Earthy (matrix) to Vitreous (crystals); Structure: Porphyritic (large crystals in fine-grained matrix); Specific Gravity: 2.8 - 3.0.
Formation & geological history
Formed through a two-stage cooling process of mafic lava: initially cooling slowly underground to form large crystals (phenocrysts), followed by a rapid surface eruption where the remaining lava cooled into a fine-grained matrix.
Uses & applications
Commonly used in construction as crushed stone or road base, for shoreline protection (riprap), and in some cases, attractive specimens are used for educational geological kits.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The 'spots' or phenocrysts in this specimen are likely pyroxene or olivine crystals.
Field identification & locations
Identify by looking for distinct dark, visible crystals embedded in a uniform, fine-grained dark grey background. Found in volcanic regions or riverbeds downstream from volcanic sites. Collectors look for high contrast between matrix and crystals.
