HearthstoneWB/Instant Return StoneNV (귀환석): Lit. "to return" 귀환 + "rock" 석 > a return stone. In the series, the stone allows a user to escape and leave the Dungeon, a valuable item that is used when in a dire situation. Sung Jin-Woo in the whole series never once used the Hearthstone/Instant Return StoneNV. He once tried to use it in the Job Change Arc to escape the Job Change Dungeon but it was knocked out of his hands and was lost in the sea of enemies, rendering him unable to leave.
This is what it say in the webtoon-version.