Banded Gneiss

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Alternating light (white/cream) and dark (grey/black) bands; Luster: Dull to vitreous; Crystal Structure: Foliated with course crystals; Cleavage: Generally poor, breaks along foliations irregularly; Specific Gravity: 2.6-2.9

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. This specimen likely originated from a protolith like granite or shale subjected to extreme heat and pressure, causing mineral recrystallization and segregation into the characteristic 'gneissic banding' or foliation visible here.

Uses & Applications

Used primarily in construction as crushed stone or road base. High-quality slabs are used for countertops, flooring, and ornamental building stone. Tumbled specimens are popular for amateur geology collections.

Geological Facts

Gneiss is among the oldest known rocks on Earth. The Acasta Gneiss in Canada is approximately 4 billion years old. The term 'gneiss' comes from an Old German word meaning 'sparkling' or 'to glint' because of the mineral inclusions.

Field Identification & Locations

Identify in the field by looking for 'gneissic banding'—distinct layers of different minerals without distinct cleavage planes like schist. Commonly found in the cores of mountain ranges or ancient continental shields. Look for alternating bands of quartz/feldspar (light) and amphibole/biotite (dark).

Identified on: 4/15/2026

Mode: Standard