igneous

Basalt (Polished pebble)

Basalt (Aphanitic mafic volcanic rock)

Hardness: 6.0 on Mohs scale; Color: Dark grey to black with light plagioclase phenocrysts; Luster: Dull to waxy when tumbled; Structure: Fine-grained (aphanitic) matrix; Specific Gravity: 2.8–3.0.

Hardness: 6
Color: Dark grey to black with light plagioclase phenocrysts
Luster: Dull to waxy when tumbled

Identified March 15, 2026More igneous →

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Physical properties

Hardness: 6.0 on Mohs scale; Color: Dark grey to black with light plagioclase phenocrysts; Luster: Dull to waxy when tumbled; Structure: Fine-grained (aphanitic) matrix; 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. Examples like this pebble are often smoothed by fluvial or marine erosion over thousands of years.

Uses & applications

Used extensively in construction as road base, railroad ballast, and concrete aggregate. Polished stones like this are used for hot stone massage, decorative landscaping, and beginner rock collections.

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 in abundance on the Moon and Mars.

Field identification & locations

Identify in the field by its high density, dark color, and lack of visible large quartz crystals. Commonly found on volcanic islands, ocean beaches, and riverbeds in volcanic regions.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral