Identification guide · Igneous Rocks
How to Identify Pegmatite
Pegmatite is identified primarily by its exceptionally large crystal size, often exceeding several centimeters, made up mainly of quartz, feldspar, and mica in a granite-like assemblage, with color and luster varying by the dominant minerals present. Hardness reflects its main constituents, generally around 6 to 7 for the feldspar and quartz. Fracture and cleavage likewise depend on which mineral within the rock is being examined. The coarse grain size alone usually distinguishes pegmatite from ordinary granite, which has much finer, more uniform crystals.
Pegmatite identification checklist
| Color | Pink, White, Gray, Multicolored (with tourmaline, etc.) | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous (large crystal faces) | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Opaque (individual crystals may be transparent) | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 6–7 | It scratches glass and a steel knife and cannot be scratched by a steel file. |
| Fracture | Uneven to conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.6–2.8 | Ordinary rock-like heft (specific gravity about 2.7), similar to quartz. |
Extra tests for Pegmatite
Glass check. Pegmatite shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Pegmatite look-alikes and how to tell them apart
Both sit near Mohs 6–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Pegmatite vs Granite →
Both sit near Mohs 6–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Pegmatite vs Diorite →
Both sit near Mohs 6–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Pegmatite vs Feldspar →
Both sit near Mohs 6–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Pegmatite identification FAQ
How can you tell if a rock is Pegmatite?
Check the color (pink, white, gray, and multicolored (with tourmaline, etc.)), the vitreous (large crystal faces) luster, a Mohs hardness of 6–7. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Pegmatite?
Pegmatite has a Mohs hardness of 6–7. It scratches glass and a steel knife and cannot be scratched by a steel file.
What is the difference between Pegmatite and Granite?
Pegmatite (Mohs 6–7, vitreous (large crystal faces) luster) and Granite (Mohs 6.5, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Pegmatite vs Granite comparison for a side-by-side table.
Is Pegmatite worth anything?
Entry-level Pegmatite specimens sell for around $1, typical collector pieces about $10, and premium examples $75 or more depending on size, quality and locality.
What color is Pegmatite?
Pegmatite is most often pink, white, gray, and multicolored (with tourmaline, etc.). Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
