Biotite Gneiss

Physical Properties

Hardness: 6-7 (overall rock); Color: Dark grey to black with vitreous/shiny flakes; Luster: Pearly to vitreous on mica surfaces; Crystal structure: Monoclinic (mica flakes) within a foliated texture; Cleavage: Perfect basal cleavage in the mica components.

Formation & Geological History

Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. Substantial heat and pressure cause the alignment and segregation of minerals into bands or 'eyes' (augen).

Uses & Applications

Used primarily in construction as crushed stone or 'road metal'. High-quality slabs are sometimes used as dimension stone for countertops or monuments. Smaller mica-rich specimens are of interest to collectors.

Geological Facts

Gneiss is among the oldest known rocks on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The 'glimmer' in this specimen comes from Biotite, often called 'black mica'.

Field Identification & Locations

Identify in the field by looking for distinct banding (foliation) and shiny, flaky dark mineral inclusions. It is commonly found in the cores of mountain ranges and eroded shield areas like the Canadian Shield or the Appalachians.

Identified on: 4/16/2026

Mode: Standard