The lump of crystal is a quest item used during the Legends' Quest. It is obtained by killing one of three knights Ranalph Devere, Irvig Senay and San Tojalon in the dungeon under Kharazi Jungle. When used with the chunk of crystal and the hunk of crystal (obtained by killing the other two knights) they form the heart crystal required to unlock a door that leads further into the dungeon to continue further on in the quest.
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.