Gneiss
Gneiss is a high-grade metamorphic rock defined by coarse banding, called gneissic foliation, in which light-colored quartz and feldspar layers alternate with darker bands rich in biotite, hornblende, or garnet. It forms under intense heat and pressure deep within the crust, often from the metamorphism of granite, sedimentary rock, or a lower-grade metamorphic precursor like schist. The word gneiss derives from an old German mining term meaning roughly bright or sparkling.
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Gneiss properties at a glance
| Type | Metamorphic Rocks |
|---|---|
| Mohs hardness | 6–7 |
| Luster | Vitreous to glimmering (mica-rich bands) |
| Fracture | Uneven, with foliated parting |
| Specific gravity | 2.65–2.75 |
| Transparency | Opaque |
| Common colors | Gray, Pink, Banded black and white, White |
| Rarity | common |
Geology & background
Gneiss forms during regional metamorphism at temperatures and pressures high enough to partially melt and physically segregate minerals into distinct compositional bands, a process most common deep within mountain belts and ancient continental cores called cratons. Some of the oldest dated rocks on Earth are gneisses, including the roughly four-billion-year-old Acasta Gneiss of Canada's Northwest Territories, and extensive gneiss terrains underlie much of the Canadian Shield, the Scottish Highlands, and the Adirondack Mountains of New York.
Rockhounds should note that gneiss can form from either igneous rock, called orthogneiss, or sedimentary rock, called paragneiss, and telling the two apart often requires examining mineral chemistry rather than hand specimen alone. The coarse, often folded banding makes gneiss a popular decorative and building stone, prized for its striking visual patterns. Collectors also look for gneiss containing garnet porphyroblasts, since large well-formed garnets embedded in a gneissic matrix are common and attractive.
How to identify Gneiss
Gneiss shows coarse, alternating bands of light minerals, quartz and feldspar, and dark minerals, biotite, hornblende, or garnet, giving it a striped or streaky appearance distinct from the finer, sheet-like foliation of schist. Luster and hardness vary by which mineral band is examined, typically 5.5 to 7 overall due to abundant quartz and feldspar. Streak is generally white since testing usually contacts the felsic bands. It is most often confused with schist and migmatite, but gneiss's coarser, less continuous banding and blockier fracture separate it from schist's more sheet-like fissility.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Gneiss is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Gneiss worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Metamorphic history study
- Decorative or building stone when suitable
- Education
Interesting facts
- The Acasta Gneiss in Canada's Northwest Territories includes rock dated to about four billion years old, among the oldest known on Earth.
- Gneiss can form from either metamorphosed granite or metamorphosed sedimentary rock, origins called orthogneiss and paragneiss respectively.
- The banded pattern in gneiss develops as heat and pressure physically segregate light and dark minerals into separate layers.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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