Some such stones are said to be scattered at Rameswaram during the Tsunami and one of them was brought to Patna to prove the evidence of Rama and the bridge.
About 4 years back, I was in Rameswaram and in one of the several hundreds of temples, I saw not one, but 12 such stones! They were of various shapes and sizes.
Any object when immersed in a liquid would displace some part of the liquid. This is so because when it is immersed in the liquid, it would occupy some of the volume that was earlier occupied by the liquid.
Any object floats in any liquid when the weight of the liquid, displaced by the fraction of the object immersed in the liquid, is equal to the entire weight of the object itself. Thus the fraction of the object immersed in the liquid depends on the relative densities of object and the liquid. For example when ice floats in water, approximately 90% of the volume of ice is immersed in water and only 10% remains outside water.