Rocks are naturally occurring solids which are composed of minerals & have been used by humans since ages. From Stone Age, rocks are used for various purposes. Also, the metals and minerals found in rock play an important role in our life. Get to know all the Greywacke Uses. We have provided you with all information about Greywacke rock here. Greywacke is defined as a dark coarse-grained sandstone rock which contains more than 15 per cent clay. Greywacke is available in beige, black, brown, colourless, cream, dark brown, green, grey, light green, light to dark grey, pink, red, white, yellow colors. The streak of a rock is the color of powder produced when it is dragged across an unweathered surface. The streak of Greywacke is white. Get to know more about Greywacke rock and characteristics of Greywacke rock in the next sections.