Gneiss (with Mica and Quartz)

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Gray, tan, and reddish-brown (likely due to iron oxidation); Luster: Sub-vitreous to pearly due to mica; Crystal structure: Coarse-grained foliated texture with distinct banding; Cleavage: Generally poor, though mica flakes exhibit perfect basal cleavage.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures and pressures. This specimen displays oxidation, suggesting exposure to water and air in a near-surface environment.

Uses & Applications

Used in construction as crushed stone or 'dimension stone' for flooring, gravestones, and countertops. It is also used as decorative landscaping rock.

Geological Facts

Gneiss is some of the oldest rock found on Earth, with the Acasta Gneiss in Canada being dated to approximately 4 billion years old. The banding occurs due to 'metamorphic differentiation' where minerals segregate into layers.

Field Identification & Locations

Identify in the field by looking for 'gneissose banding'—alternating light and dark mineral layers. It is ubiquitous in the roots of mountain ranges and ancient continental shields. For collectors, look for specimens with large garnets or distinct feldspar 'eyes' (augen).

Identified on: 4/15/2026

Mode: Standard