What is the ruling regarding ambergris found on the seashore?

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Ambergris found on the seashore belongs to the one who finds it. This is because the sea may have cast it ashore, and the default ruling concerning such items is that they are unowned, making them permissible for the finder, similar to game hunting. A narration from Sa'id mentions that the sea of Aden cast ashore an ambergris the size of a camel. People in Aden took possession of it. When a letter regarding this reached Umar ibn Abd al-Aziz, he commanded that one-fifth (Khums) of it be taken, and the remainder be given to those who found it. If they sold it to the authorities, they were to purchase it from them. Since a scale large enough to weigh the entire piece was unavailable, it was cut into two pieces for weighing. It was found to weigh six hundred rotl (pounds). One-fifth was taken as Khums, and the rest was given to the finders. The authorities then purchased it for five thousand dinars and sent it to Umar ibn Abd al-Aziz, who shortly thereafter sold it for thirty-three thousand dinars.