A heart of the sea is used to make a conduit. It is a rare aquatic-related item used as a crafting ingredient together with Nautilus Shell to make a Conduit. A conduit is made with The Heart of the Sea and Nautilus Shells. Heart of a sea can be obtained from a buried treasure. The Location of the treasure is marked on a buried treasure map, only found in ocean ruins and shipwrecks. The Heart of the Sea can only be found, not made.
It is stackable until 64 items but is not renewable. The first appearance of the Heart of the Sea is on the 1.13 version of Minecraft. Before it was named Heart of the Sea it was named Nautilus Core which later changed into Heart of the Sea.
Heart of the Sea is only found in treasure chests, and you will need a treasure map for that. Nautilus Shells are one of the items Drowneds hold and they are rare, and maps can be found on sunken shipwrecks or any sunken structure. You make a conduit by placing the heart of the sea in a middle of a table and then surrounding it with nautilus shells. It will increase your mining speed.