metamorphic
Granite / Gneiss (Felsic Metamorphic or Igneous Rock)
Gneiss (or weathered Granite)
Hardness around 6 to 7 on the Mohs scale. Light gray to tan/brown surface color due to weathering, with a granular and slightly foliated texture composed primarily of quartz, feldspar, and minor mica/amphibole. Luster is dull to slightly pearly on broken surfaces. Specific gravity approximately 2.65-2.75.
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Physical properties
Hardness around 6 to 7 on the Mohs scale. Light gray to tan/brown surface color due to weathering, with a granular and slightly foliated texture composed primarily of quartz, feldspar, and minor mica/amphibole. Luster is dull to slightly pearly on broken surfaces. Specific gravity approximately 2.65-2.75.
Formation & geological history
Formed either by the slow cooling of felsic magma deep underground (granite) or through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (gneiss), where intense heat and pressure recrystallized the minerals into distinct granular texture.
Uses & applications
Commonly used as crushed stone in road construction, concrete aggregate, landscaping rock, and dimension stone.
Geological facts
Rocks of this composition make up a significant portion of the Earth's continental crust. Over time, physical and chemical weathering wears down sharp edges, giving river stones and field cobbles their rounded appearance.
Field identification & locations
Identified in the field by its hard, scratch-resistant surface (quartz/feldspar), granular texture, and speckling of dark and light minerals. Commonly found in riverbeds, glacial tills, and mountainous igneous/metamorphic terranes worldwide.
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