The Carriage Stone is the magnum opus artifact of the long-absent Dr. Lostcarriage. Once bound to a user, the stone functions as an interplanar travel device.
Occasionally, in some bar, dungeon, or guildhall, an adventurer will find a contract board with a small basket of stones underneath it. Upon mentally accepting a contract and picking up a stone, the adventurer is teleported instantly to whatever location the contract requires. When the contract is completed, the stone becomes warm and starts vibrating gently. The hero can then return home at any time.
After an adventurer completes their first contract, the stone they picked up takes on a gold hue. Using a gold carriage stone when a specific contract has not been accepted transports its user to the Astral Lodge of whatever Company the stone has sorted them into.
Carriage stones are notoriously unreliable, and sometimes malfunction or gain new functions at exactly the wrong or right time as plot and the whims of Dr. Lostcarriage dictate.