Gneiss Pebble

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands. Luster: Dull to vitreous. Crystal structure: Medium to coarse-grained with foliation (gneissic banding). Cleavage: Generally poor, though individual minerals within have their own.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat and pressure deep in the crust. This specific specimen has been rounded by fluvial (water) or glacial erosion.

Uses & Applications

Used in construction as crushed stone for road bases and concrete aggregate. Select decorative pieces used for river rock landscaping or lapidary tumbling for hobbyists.

Geological Facts

Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is dated to about 4 billion years old. The banding is caused by the segregation of minerals during high-temperature recrystallization.

Field Identification & Locations

Identify by looking for distinct 'gneissic banding' where dark and light minerals form layered stripes. Commonly found in planetary mountain belts and glaciated riverbeds. This specimen is an 'erratic' or river pebble, found far from its source outcrop.

Identified on: 4/21/2026

Mode: Standard