metamorphic
Gneiss boulder
Banded granitoid gneiss; chiefly quartz, feldspar, and biotite/hornblende
AI-generated identification · may be incorrectTypically coarse-grained, gray to pink, with conspicuous wavy light-and-dark banding; quartz and feldspar give a vitreous to dull luster, while mica may glitter. Hardness is about 6–7 Mohs, with weak foliation and poor cleavage overall.
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Physical properties
Typically coarse-grained, gray to pink, with conspicuous wavy light-and-dark banding; quartz and feldspar give a vitreous to dull luster, while mica may glitter. Hardness is about 6–7 Mohs, with weak foliation and poor cleavage overall.
Formation & geological history
Formed when granite or other felsic rock underwent intense heat and pressure, recrystallizing and segregating minerals into compositional bands. Its protolith may be Precambrian or younger, depending on locality; the rounded shape reflects long-term weathering and transport.
Uses & applications
Used as dimension stone, paving, facing, crushed aggregate, and landscape stone; attractive polished varieties are used for countertops and architectural panels. Ordinary field boulders have little commercial value.
Geological facts
The image shows strong folded compositional banding, characteristic of gneiss rather than uniformly textured granite. Individual bands may record ancient tectonic deformation and multiple metamorphic events.
Field identification & locations
Identify it by alternating quartz/feldspar-rich pale bands and dark biotite- or hornblende-rich bands, commonly folded or contorted; a fresh surface reveals interlocking crystals. Gneiss occurs widely in ancient continental shields and mountain cores.
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