Gneiss (Banded Gneiss)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Crystal Structure: Foliated, medium to coarse-grained texture; Cleavage: Poor, but splits along bands; Specific Gravity: 2.6-2.9.

Formation & Geological History

Formed by high-grade regional metamorphism of igneous or sedimentary protoliths. Intense heat and pressure cause minerals to segregate into distinct layers. This specific specimen has been water-worn into a smooth river cobble shape.

Uses & Applications

Used in construction as crushed stone or dimension stone for countertops and flooring. Highly valued by collectors for ornamental use and as 'viewing stones' (Suiseki) due to artistic patterns.

Geological Facts

The banding in gneiss is called 'gneissic foliation.' Some of the oldest known rocks on Earth are Acasta Gneiss from Canada, dating back nearly 4 billion years.

Field Identification & Locations

Identify by distinct light and dark stripes (compositional banding) that can be wavy or contorted. Often found in riverbeds near mountainous regions with exposed metamorphic cores.

Identified on: 4/19/2026

Mode: Standard