Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, 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.
Photomicrographs of feldspathic (L) and lithic (R) greywacke. The top images are in plane-polarized light; the bottom images are in cross-polarized light. Cements fill the pore spaces.
Closeup of Pharaoh Menkaure's greywacke statue, 25th century BCE, from the Egyptian Museum in Cairo
A greywacke rock
Example of greywacke cliffs, Mangaweka, Nthrn.Manawatu, New Zealand
Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Cut slab of sandstone
Alcove in the Navajo Sandstone
Kokh-type tombs cut into the multicoloured sandstone of Petra
Sand grains of quartz with hematite coating providing an orange colour