Banded Gneiss

Hardness 6-7 on Mohs scale; consists of alternating dark (biotite/amphibole) and light (quartz/feldspar) bands; foliated texture; dull to waxy luster when water-worn; crystalline structure.

Formed through high-pressure and high-temperature regional metamorphism of igneous or sedimentary protoliths. The banding, known as gneissic layering, occurs as minerals segregate under extreme conditions during orogenic (mountain-building) events.

Used as crushed stone for road construction and railroad ballast; polished slabs are used for countertops, flooring, and decorative grave markers.

Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, dated to 4 billion years old. The bands provide a visual record of the intense tectonic forces that shaped the crust.

Identify by distinct 'zebra-like' stripes or banding that does not easily cleave. Commonly found in glaciated areas as pebbles (like this specimen) or in mountainous regions. Collectors look for unique swirl patterns caused by folding.