igneous

Scoria (Lava Rock)

Mafic extrusive volcanic rock (Scientific formula varies based on mineral content: primarily SiO2, MgO, and FeO)

Hardness: 5-6 Mohs. Color: Dark brown, black, or reddish-purple. Luster: Dull/Earthy. Structure: Highly vesicular (contain gas bubbles). Cleavage: None. Specific gravity: Low (often sinks in water, unlike pumice).

Hardness: 5-6 Mohs
Color: Dark brown, black, or reddish-purple
Luster: Dull/Earthy

Identified March 18, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of basaltic or andesitic lava rich in dissolved gas. As the lava is ejected from a volcano, the pressure drops and gas expands, creating holes (vesicles) that are frozen in place as the rock solidifies.

Uses & applications

Commonly used in landscaping (lava rocks), high-temperature insulation, drainage systems, gas grills, and as a lightweight aggregate in concrete blocks.

Geological facts

Scoria is often mistaken for pumice, but it is much denser and darker because it contains more iron and magnesium. It is the primary component of 'cinder cones,' which are steep, conical volcanic hills.

Field identification & locations

Identify by its 'Swiss cheese' appearance, dark color, and rough, abrasive texture. It is common in active volcanic regions like Hawaii, Iceland, and the American Southwest. Collectors should look for pieces with distinct, large vesicles.

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