igneous

Basalt with Quartz Infill

Aphanitic Mafic Igneous Rock with Silica (SiO2) inclusion

Hardness: 6-7 (Mohs); Color: Dark grey to black matrix with a milky white/translucent inclusion; Luster: Dull or earthy (matrix), Vitreous (quartz); Crystal Structure: Fine-grained aphanitic matrix; Specific Gravity: 2.8-3.0

Hardness: 6-7 (Mohs)
Luster: Dull or earthy (matrix), Vitreous (quartz)

Identified June 2, 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. The lighter inclusion likely formed as an amygdule, where silica-rich fluids filled a gas bubble (vesicle) post-eruption.

Uses & applications

Common basalt is used in construction as aggregate, road base, and railroad ballast. Smooth river-worn specimens like this are often kept as curiosities or used in landscaping.

Geological facts

Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. When weathered by water into smooth shapes, they are often called 'river stones' or 'beach pebbles'.

Field identification & locations

Identify by its dark, heavy, fine-grained texture and lack of visible crystals in the dark matrix. Its rounded shape indicates significant water erosion from a river or coastline.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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Dalmatian Jasper

Aplite with Arfvedsonite inclusions

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Magnetite

Iron(II,III) oxide (Fe3O4)

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