greywacke

thumb|right|Photomicrographs of feldspathic (L) and lithic (R) greywacke. The top images are in Plane polarized light|plane-polarized light; the bottom images are in [[cross-polarized light. Cements fill the pore spaces.]] thumb|Closeup of Pharaoh Menkaure's greywacke statue, 25th century BCE, from the [[Egyptian Museum in Cairo]]

Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume.