Original: 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 lithic fragments set in a compact, clay-fine matrix.
Simplified: Greywacke (German grauwacke, signifying a grey, earthy rock) is a type of sandstone.