igneous

Pegmatite

Granitic Pegmatite

Hardness: 6-7 Mohs; Color: White, gray, tan, and black; Luster: Vitreous to pearly; Crystal Structure: Very coarse-grained phaneritic; Cleavage: Good in feldspars; Specific Gravity: 2.6-2.7

Hardness: 6-7 Mohs
Color: White, gray, tan, and black
Luster: Vitreous to pearly

Identified May 23, 2026More igneous →

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Physical properties

Hardness: 6-7 Mohs; Color: White, gray, tan, and black; Luster: Vitreous to pearly; Crystal Structure: Very coarse-grained phaneritic; Cleavage: Good in feldspars; Specific Gravity: 2.6-2.7

Formation & geological history

Formed during the final stages of a magma body's crystallization. It originates from water-rich melts that allow for rapid ion migration, resulting in exceptionally large crystals. Found in intrusive igneous environments.

Uses & applications

Source of rare earth elements, industrial minerals like quartz and feldspar, and gemstones such as tourmaline, beryl, and topaz. Used in ceramics and electronics.

Geological facts

Pegmatites are famous for containing the largest crystals ever found on Earth, with some individual crystals reaching over 10 meters in length.

Field identification & locations

Identify by the presence of extremely large, interlocking crystals of quartz, feldspar, and mica. Commonly found in mountains or tectonic shield areas like the Black Hills of South Dakota or Minas Gerais, Brazil.

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