Visions of Mana

Treasure Chests will be marked on your map with a green bullseye icon, and there are well over 100 in the game to find, so don't worry too much about this achievement. It's a good idea to uncover every green bullseye though, just to be sure it isn't a Meridian or other type of collectible which have achievements for finding all of them, plus the chests will have useful items for you. I unlocked this right at the beginning of Chapter 2.