Gneiss
Image: <a rel="nofollow" class="external text" href="https://www.geograph.org.uk/profile/139">Anne Burgess</a> (CC BY-SA 2.0)
Mineral NameGneiss
Chemical FormulaVariable, composed of multiple minerals
Mohs Hardness6
Streak ColorWhite to light grey (variable)
CleavageNot applicable (rock); constituent minerals may have cleavage
FractureUneven to conchoidal
Specific Gravity2.65
Colorsgrey, white, black, pink, red, brown
TransparencyOpaque
Type LocalitySouth Uist, Hebrides, Scotland
Mineral GroupMetamorphic rock
Usesbuilding material, dimension stone, road aggregate, decorative stone
Associated Mineralsquartz, feldspar, mica, amphibole, garnet
Raritycommon
Description
A strongly foliated metamorphic rock displaying prominent alternating bands of light and dark minerals, intricately folded, with a rock hammer providing scale.
Geological Context
Gneiss forms under high-grade regional metamorphism from pre-existing igneous or sedimentary rocks, experiencing intense pressure and temperature that causes mineral segregation into bands and often complex folding, indicative of significant tectonic deformation.
