The following chapter of our Assassin's Creed: Odyssey guide contains the description of the Ainigmata Ostraka found on Petrified Islands.
There is only one Ainigmata Ostraka to be found here. It's called To the Edge of the World.
To the Edge of the World
Location of Ainigmata Ostraka: Head for Anavatos Ruin. The ostraca can be found to the right of the entrance, right beside the letter shelves.
How to solve: The item is located in Angelos Cave, on the eastern edge of the map. The cave is located among the rocks.