Gneiss (specifically Micaceous Gneiss)

Physical Properties

Hardness: 6-7 (approx); Color: Gray to light brown with sparkling silver inclusions; Luster: Dull to pearly (due to mica); Structure: Foliated with gneissic banding; Specific Gravity: 2.6-2.9

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths like granite or shale). Subjected to high temperatures and pressures deep within the Earth's crust.

Uses & Applications

Used primarily in building construction as crushed stone for road bases, concrete aggregate, and occasionally as dimension stone for architectural facings and flooring.

Geological Facts

Gneiss contains some of the oldest known crustal rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old.

Field Identification & Locations

Identify in the field by looking for 'Gneissic banding'—alternating layers of light (quartz/feldspar) and dark (biotite/amphibole) minerals. This specimen shows significant schistosity/foliation due to high mica content.

Identified on: 4/14/2026

Mode: Standard