igneous
Basalt (with Plagioclase Phenocrysts)
Porphyritic Basalt
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with white needle-like crystals, Luster: Dull or earthy matrix with vitreous inclusions, Crystal structure: Fine-grained aphanitic matrix with larger phenocrysts, Specific Gravity: 2.8–3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with white needle-like crystals, Luster: Dull or earthy matrix with vitreous inclusions, Crystal structure: Fine-grained aphanitic matrix with larger phenocrysts, Specific Gravity: 2.8–3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The white flecks (phenocrysts) began cooling slowly underground before the final eruption. Common throughout various geological eras.
Uses & applications
Commonly used as crushed stone for road base, concrete aggregate, and railroad ballast. Some porphyritic varieties are used as decorative garden stones or architectural veneers.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. The light-colored needle-like crystals in this specimen are likely plagioclase feldspar.
Field identification & locations
Identify by its dark, heavy, and fine-grained texture with distinct lath-shaped white crystals. It is ubiquitous in volcanic regions such as the Pacific Northwest, Hawaii, or Iceland.
