Looks like when two branches rub against each other for years without fusing together, it's certainly an extreme example of this though.
Hard to identify the species.
My first thought when I saw the pic was an old wasp nest cap,
if it was from 2 branches rubbing together I would expect it would be flatter or or look more concave vertically rather than circular and convex
Either way theat would make a stunning bowl or plate
I would say its a big knot, that the rest of the tree has disintegrated around allowing it to fall free.
Knots are denser and thats possibly the reason its survived.