This chapter of Assassin's Creed: Odyssey walkthrough includes a description of all Ainigmata Ostraka hidden across Korinthia. Below, you can find helpful information on locating these Ostraka: Rock and Roll and Stone Cold.
Rock and Roll
Location of Ainigmata Ostraka: Dilkos West Ramp. You can find the item on the planks lying on the sand.
Riddle Solution: Go to the Grave of Sisyphus. It is a cave located next to the large statue of Sisyphus. Inside, you will encounter many soldiers whom you have to defeat before you retrieve the item. The item itself is located inside the cave, on the table.
Location of Ainigmata Ostraka: You can find it in the Sacred Cave, within the city.
Riddle solution: Go to the area near Kephiso's Sanctuary Ruins. There, you can find a large stone skull with an open mouth. Inside it, lying on the tongue is the item you seek.