igneous

Gabbro

Gabbro (composed mainly of calcium-rich plagioclase feldspar and pyroxene)

Hardness: 6-7 on Mohs scale; Color: Dark gray to black, sometimes with a greenish tint; Luster: Dull to sub-vitreous; Crystal Structure: Phaneritic (coarse-grained, visible crystals); Specific Gravity: 2.7 to 3.3.

Hardness: 6-7 on Mohs scale
Color: Dark gray to black, sometimes with a greenish tint
Luster: Dull to sub-vitreous

Identified March 23, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow cooling of magnesium-rich and iron-rich magma deep beneath the Earth's surface (intrusive). It is the plutonic equivalent of basalt and is commonly found in the oceanic crust and ophiolite complexes.

Uses & applications

Used widely as heavy-duty construction aggregate, road base, and crushed stone. High-quality specimens are polished and sold as 'black granite' for kitchen countertops, floor tiles, and cemetery headstones.

Geological facts

Gabbro makes up a large portion of the Earth's deep oceanic crust. It is often chemically and mineralogically identical to basalt, but it has much larger crystals because it cooled slower underground.

Field identification & locations

Identify in the field by its dark color and visible interlocking crystals that lack quartz. It is commonly found in mountainous regions where tectonic uplift has exposed deep-seated rocks. Collectors should look for 'Orbicular Gabbro' for unique patterns.

Other igneous specimens

See all

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

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral