metamorphic

Orbicular gneiss (likely augen gneiss)

Quartz–feldspar–biotite gneiss; locally termed augen gneiss

AI-generated identification · may be incorrect

Dark gray, strongly foliated, with pale rounded feldspar/quartz “augen” and a medium-grained texture. Hardness is approximately 6–7 Mohs; vitreous to dull luster, no obvious cleavage as a whole, and high specific gravity around 2.7–2.9.

Identified September 21, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when pre-existing granite or volcanic rock underwent intense regional metamorphism, recrystallization, and deformation, commonly during Precambrian mountain building. The streaked fabric reflects mineral segregation and shearing.

Uses & applications

Used locally as decorative stone, facing stone, landscaping material, and polished specimen material; attractive pieces are collected rather than commonly used in jewelry.

Geological facts

The rounded pale eyes are feldspar-rich porphyroclasts, stretched and rotated within the darker matrix. This appearance can resemble orbicular granite, but the pronounced foliation supports gneiss.

Field identification & locations

Look for alternating dark biotite/amphibole-rich bands, light quartz-feldspar lenses, and a visibly banded or swirled fabric. Comparable rocks occur in Precambrian shield regions worldwide, including Scandinavia, Canada, Brazil, and the United States.