igneous
Porphyritic Basalt (River Stone)
Porphyritic Basalt
Hardness: 6.0; Color: Dark grey to greenish-grey matrix with tan and reddish mineral phenocrysts; Luster: Dull/Matte (tumbled); Crystal structure: Aphanitic (fine-grained) matrix with visible large crystals; Specific gravity: 2.8-3.0.
- Hardness
- 6
- Luster
- Dull/Matte (tumbled)
Identified More igneous →
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Physical properties
Hardness: 6.0; Color: Dark grey to greenish-grey matrix with tan and reddish mineral phenocrysts; Luster: Dull/Matte (tumbled); Crystal structure: Aphanitic (fine-grained) matrix with visible large crystals; Specific gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the surface, where larger crystals had already begun forming underground. This specific specimen has been water-worn and rounded by fluvial (river) or coastal erosion processes.
Uses & applications
Primarily used in construction as crushed stone or aggregate. In its tumbled form, it is used for landscaping, decorative garden stones, and home decor.
Geological facts
The term 'porphyritic' refers to the distinct size difference between the tiny grains in the background and the larger visible crystals (phenocrysts). This texture tells a story of a two-stage cooling process for the magma.
Field identification & locations
Identify by the contrast between a fine-grained, dark background and larger, lighter-colored mineral spots. Common in volcanic regions and riverbeds downstream from volcanic mountain ranges. Look for the smooth, oval shape indicating long-term water transport.
