Let’s review the bidding. There are 6 directions of motion, the vertical, the horizontal plane (which counts as two directions). Then there is twist, the rotational direction around the vertical axis, and rock and roll directions, the other two rotational directions. The reason there are rotational directions is because the seismic forces are frequently in the form of physical waves, like waves on the ocean passing under a boat. This is why granite’s stiffness is good for isolation, it resists rotational forces. It's mass is good for isolation because of its inertia - more force is required to move it.