Pegmatite Identification Guide
Identifying pegmatite by its exceptionally coarse interlocking crystals, granite-like mineralogy, and how it differs from granite and other coarse rocks.
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What Pegmatite Looks Like
Pegmatite is an intrusive igneous rock defined by its exceptionally coarse grain size — interlocking crystals commonly centimeters across, sometimes meters. Most pegmatites are granitic in composition: dominated by quartz, potassium feldspar, and mica (muscovite/biotite), with feldspars often pink, white, or gray. Many pegmatites host large, well-formed crystals of accessory minerals like tourmaline, beryl, garnet, topaz, and spodumene. The overall look is a light-colored, very coarse, blocky crystalline rock.
Quick visual cues
- Very large, easily visible interlocking crystals
- Light-colored (felsic): quartz + feldspar + mica
- Blocky feldspar, glassy quartz, and flashy mica plates
- Sometimes spectacular accessory gem crystals
Step-by-Step Field ID Checklist
- Judge grain size: if individual crystals are larger than ~2-3 cm and interlocking, suspect pegmatite.
- Identify the minerals: look for glassy gray quartz, blocky pink/white feldspar, and shiny mica sheets.
- Check feldspar cleavage: two cleavages near 90°, hardness ~6.
- Check quartz: no cleavage, hardness 7, conchoidal fracture, glassy.
- Peel a mica flake: mica splits into thin elastic sheets (perfect basal cleavage).
- Scan for accessory crystals: tourmaline (striated prisms), beryl (hexagonal), or garnet (rounded dodecahedra).
Key Diagnostic Tests
- Texture: the key diagnostic — extremely coarse, interlocking crystals.
- Mineral hardness: quartz 7, feldspar 6, mica 2-3 — test individual grains.
- Cleavage by mineral: feldspar (2 directions ~90°), mica (1 perfect), quartz (none).
- Overall density: moderate (~2.6-2.7 for granitic pegmatite).
- Streak: colorless minerals give white streak.
- No acid reaction (silicate, not carbonate).
Common Look-Alikes and How to Tell Them Apart
- Granite: same minerals but medium grained and even-textured; pegmatite is far coarser and often more erratic in grain size.
- Graphic granite: a pegmatite texture where quartz and feldspar intergrow in runic patterns — a subtype, not a separate rock.
- Gneiss: coarse but banded/foliated and metamorphic; pegmatite is unfoliated and igneous.
- Marble/quartzite: monomineralic and lack the quartz-feldspar-mica mix.
- Aplite: the opposite extreme — very fine-grained sugary felsic rock, often paired with pegmatite.
The single most decisive feature is the extreme coarseness of interlocking crystals combined with a granitic mineral assemblage and the absence of metamorphic banding.
Where Pegmatite Is Found
Pegmatites form from the last, water- and volatile-rich melt of crystallizing granite, allowing huge crystals to grow. They occur as dikes, veins, and pods around granite intrusions worldwide — famous gem pegmatites in Brazil (Minas Gerais), Pakistan/Afghanistan, Madagascar, California (Pala), Maine, and Namibia.
Frequently asked questions
How can you tell if a rock is pegmatite?
Pegmatite has exceptionally coarse, interlocking crystals (often centimeters or larger) of quartz, feldspar, and mica, with no metamorphic banding. The giant grain size is the defining clue.
What does pegmatite look like?
It is a light-colored, very coarse igneous rock of glassy quartz, blocky feldspar, and shiny mica, often containing large crystals of tourmaline, beryl, or garnet.
Pegmatite vs granite — what's the difference?
They share the same minerals, but granite is medium-grained and evenly textured while pegmatite is dramatically coarser with much larger, often erratic crystals.
Why does pegmatite have such large crystals?
It crystallizes from the final water- and volatile-rich melt of a cooling granite; the fluids let atoms move freely, growing exceptionally large crystals.
Pegmatite identified by the community
Recent Pegmatite specimens identified with Rock Identifier.
