Treasure Hunters are special escourtable NPC's, similar to the Merchant Escourtables, who are trying to reach their treasure map's location.
When these escourtable's become available, their locations are announced via Town Crier NPC's. In order to accept the task of escourting these NPC's to their destination, you will be asked to pay a number of gems. This amount can be 500 or less of any gem type (ie. 300 Citrines.)
Once paid, you can escourt the NPC to it's destination, where it will dig up the treasure chest for you and allow you to take your share of the treasure. These chests may contain rare items.