igneous

Granite Pegmatite

Coarse-grained Granite (SiO2 - Aluminum and Alkalies)

Hardness: 6-7 Mohs; Color: Mottle white, tan, and dark gray/green; Luster: Vitreous to pearly; Crystal structure: Phaneritic (very large crystals), often containing orthoclase, quartz, and mica; Cleavage: Variable based on mineral (perfect in micas); Specific gravity: 2.6-2.7

Hardness: 6-7 Mohs
Color: Mottle white, tan, and dark gray/green
Luster: Vitreous to pearly

Identified March 21, 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 allows for rapid crystal growth. These structures often date back to the Proterozoic or Paleozoic eras and form in deep-seated plutonic environments.

Uses & applications

Used as an industrial source for rare elements (lithium, tantalum), high-grade industrial minerals (mica and feldspar), and occasionally crushed for architectural stone or gravel. Large specimens are popular with amateur rock collectors.

Geological facts

Pegmatites are famous for growing some of the largest crystals on Earth; a single spodumene crystal in a pegmatite has been recorded at 47 feet long. They act as 'geological trash cans' where rare elements that don't fit in common minerals finally crystallize.

Field identification & locations

Identify in the field by looking for exceptionally large mineral grains (usually >1cm) compared to surrounding rock. Common in mountainous regions with exposed continental crust like the Black Hills of South Dakota or the Appalachian mountains. Tips for collectors: Check for small translucent quartz pockets which may hide 'vugs' with well-formed crystals.

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral