Flor de la Mar

The Flor de la Mar was a ship which sunk off the coast of Sumatra in the early 15th century. It was carrying a hoard of valuable treasure, which was worth "any amount of statues", according to Pirate Queen Zheng Yi Sao. This treasure included a plutonic crystal known as the Keystone. (TV: Legend of the Sea Devils)

Many years after the sinking, a ship captained by Sin Ji-Hun retrieved the treasure, including the Keystone, from the wreck. The Keystone was passed to Lei Bao, a member of his crew, and his ship with the remainder of the treasure was commandeered by the Chief Sea Devil in 1533. The legend of Ji-Hun and the treasure of the Flor de la Mar survived until at least 1807, when Zheng Yi Sao destroyed a statue keeping the Chief Sea Devil imprisoned in search of the treasure to pay off a ransom from the Black Flag Fleet. Together with Ji-Hun, the Thirteenth Doctor, Yasmin Khan, Dan Lewis, and Lei Bao's descendant Ying Ki, Zheng successfully fought several Sea Devils for possession of the Keystone and the rest of the ship's treasure. The Keystone was used to seal the Sea Devils back underwater, while the remainder of the treasure was used by Zheng to pay the ransom. (TV: Legend of the Sea Devils)

On New Year's Day 2022, the Thirteenth Doctor told Yasmin Khan and Dan Lewis that she always wondered what happened to it and its lost treasure, and suggested they should go find out. (TV: Eve of the Daleks)

Behind the scenes
In real life, the Flor de la Mar sunk in November 1511, rather than in the early 1400s (as the Thirteenth Doctor's "four centuries off" line would imply).