The stone must be very strong and durable stone not brittle without the other materials constantly mix with mix, that is, in 1 volume with a lump sum that is more than three times the length of the other side of the cube of a lump sum amounting to 20% did not.
Rock is rock solid, clean, not brittle. No other materials, additives must be mixed uniformly, ie in the first volume of a cube with a length of more than 3 times the mass of the more than 20% do not.
The rock must be clean, strong, durable, no stone fragility. Without other materials additives have been mixed uniform, that is, on 1 volume. A cube with a length of more than 3 cubes as of other side of cubes over 20%.