06/19/2026
Isn't it pretty! Many sparkle in the sunlight.
Gneiss is a high-grade, coarse-grained metamorphic rock characterized by distinct compositional banding, where minerals are segregated into alternating light-colored layers (mostly quartz and feldspar) and dark-colored layers (such as biotite and amphibole). Formed under extreme temperatures and pressures deep within the Earth's crust during regional metamorphism, it originates from pre-existing rocks like granite or shale. Unlike low-grade metamorphic rocks, gneiss does not split easily along its bands due to its highly crystalline, interlocking structure, often featuring unique eye-shaped mineral lenses known as "augen."