igneous

Basalt Cobble

Vesicular Basalt (Mafic Extrusive)

Hardness: 6.0; Color: Dark grey to black, weathering to brown; Luster: Dull/Earthy; Structure: Fine-grained/aphanitic with visible vesicles (gas bubbles); Specific Gravity: 2.8 - 3.0

Hardness: 6
Color: Dark grey to black, weathering to brown
Luster: Dull/Earthy

Identified March 22, 2026More igneous →

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Physical properties

Hardness: 6.0; Color: Dark grey to black, weathering to brown; Luster: Dull/Earthy; Structure: Fine-grained/aphanitic with visible vesicles (gas bubbles); Specific Gravity: 2.8 - 3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. This specific specimen has been rounded through fluvial (water) or glacial erosion, typically over several thousand years.

Uses & applications

Commonly used as crushed stone for road base, concrete aggregate, railroad ballast, and ground cover. High-quality denser variants are used for floor tiles or stone monuments.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is the primary rock type on the Moon and Mars.

Field identification & locations

Identify in the field by its high density, dark color, and rounded shape if found near water. Look for small holes (vesicles). It is commonly found in volcanic fields, riverbeds, and coastal areas.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

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