PUMICE
This is probably pumice, an igneous rock that has encapsulated lots of air bubbles during its formation process that is through a volcanic eruption.
Two facts about the pumice:
1. it is light in weight (you can see people holding it high easily in the video),
2. the force of the encapsulated air to go above the water overpowers the force of attraction pushing the stone to sink.
YET, and let's say i am right about pumice, it does not take away any of its spiritual significance.
NONE AT ALL.
Why not?
Because this is not a battle "science" against "true reality" (which is an idea that Western societies have entertained on wrong foundations).
Science can explain many things through reliable methods of investigations, but it still works WITHIN the sensory world, within a limited realm.
The true reality could not compete with science, on the opposite, the only way that the truth of reality may be approached by science is for science to be proving it.
We cannot oppose a bigger reality to a smaller reality within the bigger one...