igneous

Basalt with Quartz/Calcite matrix

Basalt (SiO2 with Ca/Mg/Fe silicates) on a Mineral Matrix

Hardness: 6-7 (basalt), 3-7 (matrix); Color: Dark grey to olive-green or black; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained); Specific Gravity: 2.8–3.0

Hardness: 6-7 (basalt), 3-7 (matrix)
Color: Dark grey to olive-green or black
Luster: Dull to sub-vitreous

Identified March 20, 2026More igneous →

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Physical properties

Hardness: 6-7 (basalt), 3-7 (matrix); Color: Dark grey to olive-green or black; Luster: Dull to sub-vitreous; 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 exposed at or very near the surface. The lighter matrix represents secondary mineral deposition or a contact zone during volcanic activity.

Uses & applications

Commonly used as aggregate in construction, road base, and railroad ballast. Decorative specimens are used in landscaping or as aquarium stones.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is similar in composition to the rocks found on the Moon and Mars.

Field identification & locations

Identify in the field by its dark, heavy, and fine-grained texture. It often weathers to a rusty brown due to iron oxidation. Commonly found in volcanic fields, riverbeds, and coastal areas.

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

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