It's expired now, so if you don't have one you'll have to wait and hope it is re-released at some point.
Try Iceberg + River. It can get you a century, at the same time as having a chance to get an aquamarine. But it will only work now during "Bring 'em Back" because the when the Century dragon comes back it might not be in the month of Aquamarine. Good luck! 😄