Igneous

Basalt (Vesicular)

Vesicular Basalt

Hardness: 5-6 (Mohs scale), Color: Dark gray to black, Luster: Dull or earthy, Crystal Structure: Aphanitic (fine-grained), Cleavage: None (conchoidal fracture), Specific Gravity: 2.8 - 3.0. Features numerous small pits or gas bubbles (vesicles).

Identified March 21, 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 very near the Earth's surface. The 'pits' are formed by gas bubbles escaping as the rock solidifies. Most specimens are from the Cenozoic era, though basalt has been forming throughout Earth's history.

Uses & applications

Used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. In landscaping, vesicular varieties are often sold as 'lava rock.' Occasionally used for stone tools or thermal insulation (stone wool).

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 common on the Moon (the dark 'maria' plains) and Mars.

Field identification & locations

Identify in the field by its dark color, heavy weight relative to its size, and the presence of small holes (vesicles). Commonly found in volcanic regions, oceanic islands, and mid-ocean ridges. Collectors look for 'vugs' within the vesicles that may contain secondary minerals like zeolites or quartz.

Other igneous specimens

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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