In the rule “Tools” replace “A Monk may use a tool that they are holding at any time by spending its turn cost in turns and applying its effect. The valid tools are held in this rule” with

The Turn Cost of a tool may be a non-negative integer or the word “Action”. If it is an integer, A Monk may use a tool that they are holding at any time by spending its turn cost in turns and applying its effect. If it is Action, a Monk may do a Tile Action to use a tool and apply its effect

replace the turn cost of the Grinding Stone with “Action” and the effect with “Change the colour of a white tile to match that of a tile which is which is distance 2 from it. This action is known as Grinding”