igneous

Basalt River Pebble

Basalt (Mafic volcanic rock)

Hardness: 5-6 (Mohs scale); Color: Dark gray to black (weathered to light gray); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.

Hardness: 5-6 (Mohs scale)
Color: Dark gray to black (weathered to light gray)
Luster: Dull/Earthy

Identified March 23, 2026More igneous →

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Physical properties

Hardness: 5-6 (Mohs scale); Color: Dark gray to black (weathered to light gray); Luster: Dull/Earthy; Crystal structure: Aphanitic (fine-grained); Specific gravity: 2.8–3.0.

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. This specimen has been rounded and smoothed through 'attrition' in a high-energy fluvial (river) or beach environment over thousands of years.

Uses & applications

Commonly used in construction as aggregate, for road base, in high-strength concrete, and for decorative landscaping. Smooth river basalt is also popular in 'hot stone' massage therapy.

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.

Field identification & locations

Identify in the field by its uniform dark color, heavy weight for its size, and the absence of visible large crystals. Commonly found in areas of past volcanic activity or downstream in riverbeds from volcanic highlands.

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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