Rock Identifier
Porphyritic Basalt (with Quartz or Calcite inclusion) (Porphyritic Basalt (Matrix: SiO2/MgO/FeO, Phenocryst: likely Quartz or Calcite)) — igneous
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.

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