Rolling Stone is a discovery in Skies of Arcadia Legends.
In the Dark Rift, there is a boulder that defies the power of the storm within which it lives. Scientists have tried to study the rock to figure out how it floats, but it moves with such speed that any ship that gets too close is pulled into the depths.
The Rolling Stone has an large movement range and can be very difficult to locate on a whim or without otherwise using trickery. The track the object takes is simple: it travels clockwise in the Lower Sky just inside the standard "white" sky rifts that form a circle around the central dark rift that the player can enter as a "ship dungeon". It is so difficult to find because, not only is this a huge track to search, but as it travels this path it passes through "black" sky rifts - which the player cannot penetrate - no less than 4 times.
An easy way to find the discovery is to first locate Ryu-kan's Island, enter it and promptly depart. This generally resets all moving discoveries to their "spawn" locations - which in the case of the Rolling Stone is nearly due north of Ryu-Kan's Isle. (Another similar tactic is to simply save and reset the game while flying nearby Ryu-kan's). After doing this, drop into Lower Sky and immediately travel north and the Rolling Stone should become not only visible, but obvious, as a rather sizable spinning boulder moving across your field of view on the right side.
The actual "spawn" point for the Rolling Stone is just south of the "black" sky rift located to the north of Ryu-kan's Isle, near the intersection of the black rift and the circular white rift. It travels surprisingly fast once in motion, so finding it as it spawns or shortly after is really the best option available.