Sandstone is a sedimentary rock composed of sand grains (often quartz) bound together within a matrix of silt- or clay-sized particles.  The stone’s abrasive qualities make it ideal for grinding stones, although the relative lack of silica in the matrix makes it difficult to flake in a controlled way.  Harder and fine-grained varieties of sandstone have long been shaped by direct and indirect percussion flaking into building stones, or shaped by grinding into sculptures.  Silicified sandstone can be flaked into tools if there is sufficient silica content in the matrix.