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Greywacke

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

Greywacke or graywacke (German grauwacke, signifying a grey, earthy rock) 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.

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