Abstract: For blocking or polishing a natural or synthetic stone having a Mohs hardness greater than about 5, a blocking head has a body, a mounting member which can move vertically with respect to the body, and a rotary spindle driven by a motor. The mounting member is carried by a leaf spring whose position is altered by a parallel linkage whose arm is driven by a motor. In this way, the spring can for soft landing apply an upwards bias opposing the gravitational force of the mass of the mounting member. When the stone is in contact with the scaife, the deflection of the spring measures the force on the stone, and this deflection is detected by an LVDT. Subsequently, the spring can apply a downwards bias to augment the working force. A microprocessor is responsive to the output of the LVDT and controls the motor so that the force applied to the stone complies with a predetermined program.