Iron concretion with a ball of pitch inside from the wreck of the East Indiaman 't Vliegend Hart

Presumably the object is a ball of pitch. Pitch was obtained from tar distillation. Boiling pitch that falls into water takes on a round shape. Pitch was part of harpou. Harpou is a mixture of linseed oil, resin, sulfur, and fat. Mixed with pitch, it was used for caulking. Caulking is the process of making the seam between, for example, two planks watertight. Tar, pitch.