igneous
Porphyritic Basalt
Basalt with Feldspar Phenocrysts
Hardness: 6.0; Color: Dark grey to greenish-grey matrix with tan/pink crystals; Luster: Dull (matrix) to vitreous (phenocrysts); Crystal structure: Aphanitic (fine-grained) with larger phenocrysts; Cleavage: None visible; Specific Gravity: 2.8-3.0
- Hardness
- 6
- Color
- Dark grey to greenish-grey matrix with tan/pink crystals
- Luster
- Dull (matrix) to vitreous (phenocrysts)
Identified More igneous →
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Physical properties
Hardness: 6.0; Color: Dark grey to greenish-grey matrix with tan/pink crystals; Luster: Dull (matrix) to vitreous (phenocrysts); Crystal structure: Aphanitic (fine-grained) with larger phenocrysts; Cleavage: None visible; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the surface. The larger crystals (phenocrysts) formed slowly underground before being erupted within the magma. These are often found as glacial erratics or river stones.
Uses & applications
Commonly used as crushed stone for aggregate in road construction, concrete, and railroad ballast. Smooth river-worn specimens are used in landscaping or as decorative pebbles.
Geological facts
This specific specimen is likely a glacial erratic, smoothed by ice or water transport. The contrast between the fine-grained matrix and larger crystals provides a visual record of a two-stage cooling history.
Field identification & locations
Identify in the field by looking for a dense, dark, heavy rock with distinct visible crystal spots. Common in volcanic regions and areas with glacial deposits such as the Great Lakes region or the Pacific Northwest.
