I been searching everywhere, I heard you can find in in Couriway City in the middle mini waterfall but I don't think it's true...I tried though. I already upgraded my Mega Ring at Anistar City and beat the game...
>You can find this one at night. It's in the southeastern section of Couriway Town. There are three small waterfalls and this stone is in the middle one.
It's only found at night, so that's probably why you couldn't find it.
That's strange. It should be there in the middle waterfall. Its only there between 8-9PM. Maybe you didn't check between those times.