Celtic stone

When it is spun in one direction, it spins freely without stopping. When spun in the other direction, it slows down rapidly, begins to rock back and forth wildly, and then spins in the opposite direction.

Celts rock has to have two curves at the bottom, a long smooth lengthwise one, and a sharper one across the width. Mass has to unevenly distributed along these two curves. The interplay of these curves and the mass results in an uneven application of frictional force. One direction has to fight against more friction than the other.