igneous

Vesicular Basalt (commonly mistaken for Slag)

Scoriaceous Basalt (composed primarily of Plagioclase Feldspar and Pyroxene)

Hardness: 5-6 (Mohs scale); Color: Dark grey, black, or reddish-brown; Luster: Dull to vitreous (glassy) in vesicles; Structure: Aphanitic (fine-grained) with large vesicles; Specific Gravity: 2.8-3.0

Hardness: 5-6 (Mohs scale)
Color: Dark grey, black, or reddish-brown
Luster: Dull to vitreous (glassy) in vesicles

Identified March 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of magnesium-rich and iron-rich lava. The holes (vesicles) are created by gas bubbles that were trapped in the magma as it solidified near the Earth's surface.

Uses & applications

Used in construction as road aggregate, railroad ballast, and ground cover in landscaping. Highly porous varieties like scoria are used in gas grills and as lightweight concrete additives.

Geological facts

Vesicular basalt is the most common rock on the surface of the Moon (Lunar Maria) and covers a significant portion of the Earth's ocean floor. If the vesicles fill with minerals like quartz or calcite later, it becomes 'amygdaloidal basalt'.

Field identification & locations

Identify in the field by its high density relative to its holey appearance and dark color. Common in volcanic regions like Iceland, Hawaii, and the Pacific Northwest. Note: If the specimen has circular gas bubbles and a glassy/metallic flow texture, it may be industrial iron-ore slag.

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