Location: This quest can be found in Gondolia.
Objective: Search for the Legendary Sage’s Stone
To find the Sage’s Stone, head to Nautica and give the queen a visit. Talk to her and request to be turned back into a fish then exit the castle. Make your way for the sunken ship towards the southern side of town and look for the shelf above the front of the ship as shown in the image below. Collect the gold shiny object on the shelf to acquire the Sage’s Stone, then return to the old adventurer in Gondolia and talk with him to complete the quest.
Reward: Making the Most of Mythril