igneous
Porphyritic Basalt (with Quartz or Calcite inclusion)
Porphyritic Basalt (Matrix: SiO2/MgO/FeO, Phenocryst: likely Quartz or Calcite)
Hardness: 5-7 (matrix) / 7 (quartz inclusions); Color: Dull black to dark grey matrix with white inclusions; Luster: Earthy (matrix) to Vitreous (crystals); Structure: Fine-grained aphanitic matrix with larger visible crystals (porphyritic texture).
- Hardness
- 5-7 (matrix) / 7 (quartz inclusions)
- Color
- Dull black to dark grey matrix with white inclusions
- Luster
- Earthy (matrix) to Vitreous (crystals)
Identified More igneous →
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Physical properties
Hardness: 5-7 (matrix) / 7 (quartz inclusions); Color: Dull black to dark grey matrix with white inclusions; Luster: Earthy (matrix) to Vitreous (crystals); Structure: Fine-grained aphanitic matrix with larger visible crystals (porphyritic texture).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The larger white inclusions formed earlier in the cooling process (phenocrysts) or later through secondary mineral precipitation in gas bubbles (amygdules).
Uses & applications
Commonly used as aggregate in construction, road base, and railroad ballast. Decorative specimens are used in landscaping or for geological study.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Porphyritic textures tell a story of two-stage cooling: slow cooling deep underground followed by rapid cooling during an eruption.
Field identification & locations
Identify in the field by its dark, heavy, fine-grained mass containing distinct lighter-colored crystal 'spots'. Found in volcanic regions like the Pacific Northwest, Hawaii, or Iceland. Collectors should look for sharp contrasts between the matrix and inclusions.
