igneous

Granitic Pegmatite

Pegmatite (composed primarily of Quartz, Feldspar, and Muscovite mica)

Hardness: 6-7 (Mohs scale); Color: White, cream, tan with silvery-grey flakes; Luster: Vitreous to pearly; Crystal structure: Macro-crystalline (very large crystals); Cleavage: Perfect in one direction for the mica components; Specific gravity: 2.6-2.7

Hardness: 6-7 (Mohs scale)
Color: White, cream, tan with silvery-grey flakes
Luster: Vitreous to pearly

Identified February 25, 2026More igneous →

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Physical properties

Formation & geological history

Formed during the final stages of a magma chamber's crystallization when high water content allow large crystals to grow rapidly. These are typically associated with batholiths and found in ancient continental crust.

Uses & applications

Used as a source of rare minerals and gemstones; also used in ceramics, glass making, and decorative architectural stone.

Geological facts

Pegmatites are famous for containing some of the largest crystals ever recorded, sometimes exceeding 10 meters in length. The silvery flakes seen in the specimen are likely Muscovite mica.

Field identification & locations

Identify in the field by exceptionally large, interlocking crystals of quartz and feldspar. Found in mountainous or shield regions like the Black Hills (USA) or Minas Gerais (Brazil). Collectors look for well-formed mica plates or embedded gems.

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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)

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Sodalite

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

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Magnetite

Magnetite (Fe3O4)

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