Use a heat source and dimple the walls of the tank, be careful not to puncture, just disrupt the smoothness to introduce irregularity to the inner walls.
You need to add some fins or other disruption inside the jar in order to get the tumbling action. Try gluing some small nuts to the inside of the jar. Perhaps a few rows running from the mouth of the jar to the base.
@SureshBopitiya no sir, 3/4 way full always, water just below the level of rock, this will allow the rocks to not all fall down to the bottom and stay there, the idea is that the rocks roll on top of each other. And think of sand as glass, about the same hardness, so any rocks harder than that and the sand won't do anything to them except break down the sand
