I totally agree with these two response, slow and careful is the key. It really depends on the type of rock it is attached to. If it is very porous and the foot is deep in a hole, try to get a thin screwdriver into an adjacent hole and widen it out so you can get under the foot and break out the rock the foot is attached to. If the rock is smooth or it is attached to glass then you may be able to gently remove it with your fingers, while wearing a glove.