The heart crystal is a quest item used in the Legends' Quest. It's created by killing the three knights Ranalph Devere, Irvig Senay and San Tojalon in the dungeon under Kharazi Jungle to obtain a lump of crystal, a chunk of crystal and a hunk of crystal, which can then be combined to form the heart crystal, by using them on the furnace. Once combined, the crystal must be activated by using it on the mossy rock in the caves, before it can be used on the recess in order to fill it, which deactivates the Shimmering fields.
It is not possible to obtain the heart fragments after completing the quest; killing the knights again does not provide the player with the fragments. However, duplicate parts of the crystal can be obtained during the quest by utilising the drop trick. If the player does this, they are able to create another (albeit useless) heart crystal after completing the quest. Attempting to use an activated heart crystal on the recess next to the shimmering field after completing the quest gives a chatbox message The cave is already open to you.
See the Replenishing the pure water section of the Legends' Quest guide.