17 Carat Diamond
The 17 Carat Diamond is an in-game convenience feature used for high-volume Zeny transfer.
The item is traded by the NPC called Kafra Employee.
A player can purchase a 17 Carat Diamond for 101,000,000 Zeny (101m). The item can then be traded freely between players and used as a payment.
A player who receives a 17 Carat Diamond can trade it back to the same NPC for 99,000,000 Zeny (99m).
This is the safest way to make transactions over 1b, as GMs will not get involved in player-made trades.
Kafra Employee NPC can be found in prontera.