Stubb suspects that the Rosebud’s whale carcasses contain ambergris.
The Rosebud, whose French name is Bouton de Rose, has two already-dead whales whose blubber and oil appear to be of little value. Stubb nevertheless believes the carcasses may contain ambergris, a rare and valuable material associated with sperm whales. He tricks the ship’s crew into releasing the whales and is proved right, although he recovers only part of the substance before Ahab summons him back.
Ambergris is not the same as spermaceti. Spermaceti comes from the sperm whale’s head and was widely used in candles and lubricants; ambergris forms in the digestive system and was historically valued in perfumery. The episode also shows Stubb’s practical knowledge of whaling and his willingness to exploit an opportunity, even when the catch initially looks worthless.