igneous

Basalt River Cobble

Basalt (Aphanitic Mafic Volcanic Rock)

Hardness: 6 (Mohs scale). Color: Dark grey to charcoal or greenish-black. Luster: Dull/Matte. Structure: Fine-grained (aphanitic) with a smooth tide-worn or river-worn texture. Cleavage: None. Specific Gravity: 2.8–3.0.

Hardness: 6 (Mohs scale)
Color: Dark grey to charcoal or greenish-black
Luster: Dull/Matte

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. This specimen has been rounded into a cobble by water erosion (alluvial process), likely within the last few thousand years, though the rock itself could be millions of years old.

Uses & applications

Common basalt is used in construction as aggregate for concrete and asphalt, railroad ballast, and landscaping. Smooth river stones are popular in 'hot stone' massage therapy and decorative garden masonry.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars. When basalt cools slowly in thick layers, it can form unique hexagonal columns like those at the Giant's Causeway.

Field identification & locations

Identify in the field by its dark, uniform color, heavy weight relative to size, and lack of visible large crystals. It is commonly found in volcanic regions, riverbeds, and coastal beaches. This specimen shows a characteristic 'tumble' finish from natural water movement.

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

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Magnetite

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