Throughout the game are some giant clams which hold one of two items. Either this will be a treasure chest or a Imparite Crystal. When you approach a clam wait till it clams up inront of you to ensure it does not fall on your minions. After it opens direct your minions to the item within the clam and they will retrieve it. You will do this.more than 3 times through the game but upon your third time the achievement will unlock