igneous

Pegmatite (Granite Pegmatite)

Intrusive Igneous Rock (Granite Pegmatite)

Hardness: 6-7 (Mohs scale), Color: White, gray, and dark brown to black, Luster: Vitreous to pearly, Crystal structure: Large phaneritic/porphyritic crystals, Cleavage: Varies by mineral constituent, Specific gravity: 2.6-2.7

Identified January 12, 2026More igneous →

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Physical properties

Formation & geological history

Formed by the slow cooling of water-rich magma in the final stages of crystallization of an intrusive igneous body. The presence of water allows for the growth of exceptionally large crystals.

Uses & applications

Used as a source of rare minerals such as spodumene, lepidolite, and tourmaline. Also used in construction as crushed stone and for architectural stone decorative pieces.

Geological facts

Pegmatites are famous for containing some of the largest crystals in the world, sometimes reaching several meters in length. They are the primary source of many gemstones.

Field identification & locations

Identify in the field by looking for exceptionally large mineral grains (over 1 cm) within a granitic matrix. Commonly found in shield areas of continents like the Black Hills of South Dakota or the Scandinavian Shield.

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