indicated at 42 and is dried in the process indicated at 43 the process of screening washing and drying the granite particles is of course well known and does not need description herein the drying is important or the materials will not effectively bond the supply 40 is selected from particles which are rounded that is they have been agitated sufficiently to remove sharp corners and again this is a process well known in the art the materials 1 and 2 are therefore comprised of substantially wholly granite which has been screened for the particles lie in the range 8 to 30 mesh in effect two or more particle sizes are then selected for the materials 1 and 2 in material 1 is selected to have a larger mesh size than the material 2 the amount of the particles from materials 1 and 2 is selected so that the first material having the larger particles forms a portion in the range 18 to 37 of the total finished mixture and preferably of the order of 28 the amount of the material 2 is arranged so that it lies in the range 53 to 75 of the total finished mixture and preferably of the order of 66 the binder is non toxic so that it cannot damage a student or child if inadvertently inhaled ingested or otherwise absorbed sodium silicate provides a suitable binder and can be supplied in the range 1 75 to 7 and preferably of the.