Rock Identifier

igneous

Basalt Pebble

Basalt (Aphanitic Mafic Volcanic Rock)

Hardness: 6 on the Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained, invisible to the naked eye); Cleavage: None; Specific gravity: 2.8 - 3.0.

Hardness: 6 on the Mohs scale
Color: Dark grey to black
Luster: Dull to sub-metallic

Identified March 7, 2026More igneous →

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Physical properties

Hardness: 6 on the Mohs scale; Color: Dark grey to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained, invisible to the naked eye); Cleavage: None; 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. Basalt is the most common rock type on the Earth's crust, found in oceanic plates and volcanic regions.

Uses & applications

Commonly used in construction as aggregate for concrete and asphalt, as road base, and for railroad ballast. High-quality smooth pebbles are often sold for decorative landscaping, aquariums, and hot stone massage therapy.

Geological facts

Basalt also makes up the majority of the ocean floor and has been found on the Moon and Mars. When basalt cools slowly in thick layers, it can form striking hexagonal columns, such as those at the Giant's Causeway.

Field identification & locations

Identify in the field by its high density (heavy for its size), dark color, and fine-grained texture. It often feels slightly gritty despite a polished appearance. Commonly found along coastlines, riverbeds, and volcanic fields.

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Dark Igneous Rock (Likely Basalt or Diabase)

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Magnetite (Iron(II,III) oxide (Fe3O4)) — mineral

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Sodalite (Sodalite - Na8(Al6Si6O24)Cl2) — mineral

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Magnetite (Magnetite (Fe3O4)) — mineral

Magnetite

Magnetite (Fe3O4)