igneous
Granitic Pegmatite
Granite Pegmatite (coarse-grained intrusive igneous rock)
Hardness: 6-7 (Mohs); Color: White to light grey matrix with black tourmaline/biotite inclusions; Luster: Vitreous to pearly; Crystal structure: Macro-crystalline (phaneritic); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 (Mohs)
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 6-7 (Mohs); Color: White to light grey matrix with black tourmaline/biotite inclusions; Luster: Vitreous to pearly; Crystal structure: Macro-crystalline (phaneritic); Specific Gravity: 2.6-2.7
Formation & geological history
Formed during the final stages of a magma's crystallization, where high water content allows for the growth of exceptionally large crystals. Typically found in batholiths or dikes.
Uses & applications
Source of rare earth elements, industrial minerals like feldspar and mica, and host rock for gemstones such as tourmaline, beryl, and spodumene.
Geological facts
Pegmatites contains the largest crystals ever recorded on Earth, sometimes reaching over 10 meters in length. This specimen specifically shows Schorl (black tourmaline) embedded in a quartz/feldspar matrix.
Field identification & locations
Identify by the unusually large grain size compared to normal granite and the presence of accessor minerals like black tourmaline. Commonly found in mountainous regions with ancient crystalline basement rocks like the Appalachians or the Black Hills.
