1) Start a new game.
2) Play until after the cinematic where the Helmaroc King drops Tetra into the woods atop Outset Island.
3) Stand in the Northwest corner of Aryll's Lookout and face Northwest.
4) Take out your Telescope and look around in the sky in front of and above you.
You should be able to find a small grey speck in the sky (you can't see it without your Telescope), zooming in on it will reveal it to be one of the stones the pirates were catapulting.