igneous
Basalt with Quartz/Calcite Vein
Aphanitic Mafic Volcanic Rock with Quartz (SiO2) or Calcite (CaCO3) Vein
Hardness: 5.5 to 7 (matrix is ~6, vein quartz is 7). Color: Dark grey to black matrix with a lighter greyish-green to white quartz/calcite vein. Luster: Dull to earthy matrix with vitreous vein. Texture: Fine-grained (aphanitic) matrix, crystalline vein material.
- Hardness
- 5
- Luster
- Dull to earthy matrix with vitreous vein
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Physical properties
Hardness: 5.5 to 7 (matrix is ~6, vein quartz is 7). Color: Dark grey to black matrix with a lighter greyish-green to white quartz/calcite vein. Luster: Dull to earthy matrix with vitreous vein. Texture: Fine-grained (aphanitic) matrix, crystalline vein material.
Formation & geological history
Formed from the rapid cooling of basaltic lava rich in magnesium and iron at or near Earth's surface. The lighter central band formed later as hydrothermally enriched fluids deposited quartz or calcite in fractures within the basalt pebble during low-grade metamorphism or hydrothermal activity.
Uses & applications
Basalt is primarily used in construction as crushed aggregate, road base, and concrete. Small beach pebbles like this specimen are kept as decorative stones, pocket stones, or landscaping rocks.
Geological facts
Veined basalt pebbles are common on beaches near subduction zones or volcanic coasts. Water and geothermal activity crack the dark lava rock and fill the fissures with mineral-rich fluids, creating distinct lighter lines or bands.
Field identification & locations
Identify by the stark contrast between the fine-grained, dark mafic matrix and the lighter-colored mineral vein running through it. Found frequently on marine and river beaches in volcanic or tectonic regions such as the US Pacific Northwest, California coast, Iceland, and Hawaii.
