River Stone Mystery

[hannjohn]

I found this very beautiful stone on the banks of an Australian (VIC) river. (Merrica River) It took me by surprise as it was stuck in a root and is perfectly smooth and egg shaped. Does anyone have any ideas of how this was created? And what stone it is? Our theory was that the roots grew around the stone, it was exposed and then the water smoothed it into shape. Or the rock was originally a larger shape.  It was able to spin around inside where it was sitting. Any ideas welcomed. 

[GotAU?]

That’s a beautiful agate. I think it was more likely polished by someone and left there. The wood is much softer than the stone and not really growing around it, plus the force of water required to deposit a stone that high above the waterline would have easily removed that tree.

What a beautiful stone though, nice find!