Pegmatite
Very coarse-grained intrusive igneous rock (often granitic)
An exceptionally coarse-grained igneous rock, often granitic, famous for hosting large crystals and many gemstones.
- Mohs hardness
- 6-7
- Color
- light, variable; pink, grey, white with colorful accessory minerals
- Type
- igneous
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Overview
Pegmatite is an exceptionally coarse-grained intrusive igneous rock, most commonly granitic in composition, defined by interlocking crystals that are typically larger than a few centimeters and can reach meters in size. It is composed largely of quartz, feldspar, and mica, often with rare and gem-bearing accessory minerals.
Pegmatites are famous among collectors and miners because they host outstanding crystals of tourmaline, beryl (including aquamarine and morganite), topaz, spodumene, garnet, and many rare element minerals.
A distinctive variety, graphic granite, shows intergrown quartz and feldspar resembling ancient script.
Formation & geology
Pegmatite forms in the final stages of magma crystallization, when a granitic melt has become enriched in water, volatiles, and incompatible rare elements. This fluid-rich residual melt allows ions to migrate easily, so a few crystals grow very large rather than many small ones.
Pegmatites typically occur as dikes, veins, and pods within or around granite plutons. The volatile-rich environment concentrates elements like lithium, boron, beryllium, and rare earths, producing gem and rare-mineral pockets. Famous pegmatite districts include Brazil, Madagascar, Pakistan, and the United States (e.g., California and Maine).
How to identify it
Identify pegmatite by its extremely coarse, interlocking crystals, often centimeters or larger, composed mainly of quartz, feldspar, and mica. Overall hardness is about 6-7.
Distinguish it from ordinary granite, which is coarse but with much smaller, more uniform crystals. The presence of giant individual crystals or colorful gem minerals (tourmaline, beryl, mica books) is a strong clue.
Graphic granite, with its rune-like quartz-feldspar intergrowths, is a recognizable pegmatitic texture. The sheer crystal size is the most diagnostic feature.
Uses & significance
Pegmatites are the world's premier source of gem crystals, including tourmaline, aquamarine, morganite, topaz, and kunzite, prized by collectors and the jewelry trade. They are also mined for industrial minerals: feldspar for ceramics and glass, sheet mica for electronics, and quartz.
They are major economic sources of rare elements such as lithium (from spodumene and petalite), beryllium, tantalum, niobium, cesium, and rare earths, important for batteries and high-tech industries.
Graphic granite and large crystal specimens are also valued display and ornamental pieces.
Frequently asked questions
What makes pegmatite different from granite?
Pegmatite has the same composition but far larger, interlocking crystals, formed from a water- and volatile-rich residual melt.
Why are gemstones found in pegmatites?
Their volatile-rich melts concentrate rare elements and allow large crystals to grow, producing tourmaline, beryl, topaz, and more.
What is graphic granite?
It is a pegmatitic texture of intergrown quartz and feldspar that resembles angular ancient writing or runes.
Why are pegmatites mined for lithium?
They host lithium minerals like spodumene and petalite, making them an important source of lithium for batteries.
Pegmatite guides
In-depth guides for identifying, valuing, and understanding Pegmatite.
Pegmatite identified by the community
Real specimens identified with Rock Identifier.
