metamorphic

Polished gneiss or migmatitic gneiss cobble

Banded quartz–feldspar–biotite gneiss; compositionally possibly migmatitic

AI-generated identification · may be incorrect

Typically quartz, feldspar, biotite/hornblende, and minor iron oxides; Mohs hardness about 6–7, with a granular texture, nonmetallic luster, and strong light–dark compositional banding. The rounded surface is water-worn or polished, and the pale patches may be quartzofeldspathic veins or leucosome.

Identified September 18, 2026More metamorphic →

Explore Polished gneiss or migmatitic gneiss cobble in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed when preexisting igneous or sedimentary rock underwent high-grade metamorphism, commonly during Precambrian to Paleozoic mountain building; migmatitic varieties partly melted under extreme heat and pressure. The specimen was later eroded into a cobble.

Uses & applications

Used as dimension stone, decorative aggregate, landscaping stone, and architectural facing; attractive polished pieces are collected as ornamental specimens.

Geological facts

Gneissic banding records mineral segregation and intense deformation deep within the crust; contrasting pink-tan and black layers commonly reflect feldspar/quartz versus biotite-rich material.

Field identification & locations

Identify it by alternating light and dark mineral bands, interlocking grains, and lack of vesicles or glassy texture; common in glacial deposits, river gravels, and metamorphic belts worldwide. Exact identification requires a fresh surface or thin section.