Igneous

Polished Basalt

Basalt (Mafic volcanic rock)

Hardness: 6-7 on the Mohs scale; Color: Black, charcoal gray, or dark green; Luster: Dull to sub-metallic (shiny when polished); Crystal structure: Aphanitic (fine-grained/microscopic); Cleavage: None; Specific Gravity: 2.8-3.0.

Hardness: 6-7 on the Mohs scale
Color: Black, charcoal gray, or dark green
Luster: Dull to sub-metallic (shiny when polished)

Identified March 14, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. These specimens specifically show signs of fluvial or mechanical tumbling, smoothing the naturally angular volcanic glass into rounded pebbles.

Uses & applications

Commonly used in landscaping (river rocks), decorative masonry, hot stone massage therapy, and industrial construction aggregates. High-quality polished versions are used for aquarium decor and zen gardens.

Geological facts

Basalt is the most common rock in the Earth's crust and makes up most of the ocean floor. It is also found on the Moon and Mars, where large basaltic plains are known as 'maria'.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and lack of visible crystals. Commonly found in volcanic regions, near oceanic ridges, and riverbeds downstream from volcanic sources.

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