This chapter of our guide for Assassin's Creed: Odyssey contains descriptions of all Ainigmata Ostraka that can be found on Silver Islands archipelago - Delos and Mykonos are part of it.
Delos, the western part of Silver Islands archipelago, has two Ainigmata Ostraka puzzles - they are called Needle in a Haystack and Grave Discovery.
Needle in a Haystack
The location of this Ainigmata Ostraka: Go to the Sunken Wreck of Datis. The tablet is on the ship's wreck.
Puzzle solution: Go to the farm located in the southern part of the island. The item is hidden in the haystack.
The location of this Ainigmata Ostraka: Go to the ruins presented in the picture above. The item is near the table.
Puzzle solution: Go to the Grand Temple of Apollo. The entrance to a cave is behind the statue. The item is at the end of that cave - among apples lying on the ground.
There is only one Ainigmata Ostraka on Mykonos. This one is called Marbled Morale. You can find it in the biggest cave on the island.
The location of this Ainigmata Ostraka: Go to Porphyrion Cave. The tablet is on the table located on the highest level.
Puzzle solution: Go to the quarry located in the southern part of the island. Reach the bottom. The item is among corpses of slaves.