Gate Stone

Gate Stones are rare items that allow one person to leave a dungeon at any time.

Appearance
Gate Stones are small, translucent blue stones set inside what appears to be a frame of black material, possibly stone or metal. This frame caps the stone at the smaller ends, and thin strips running vertically down the long, cuboidal stone connect the two end caps.

Effects
Using a Gate Stone allows one person, whether Awakened or not, to leave a dungeon at any time. It must be used near the dungeon's entrance gate. Presumably, it works by temporarily opening the entrance gate long enough and closes it after one person has gone through.

Gate Stones are single-use items and only work for one person. They are vanishingly rare and incredibly precious; 3 years after the dungeons appeared, there were only 22 in the entirety of South Korea, most likely in the hands of upper rankers such as guild leaders, and it was near impossible to even lay your eyes on one if you were low-ranked or Unawakened.

Original timeline, pre-regression
Han Yoohyun offers Han Yoojin a Gate Stone with which to leave the D-rank dungeon they are inside. Though Han Yoojin initially refuses, he takes it after Han Yoohyun claims he has another Gate Stone for his own use.

New timeline, post-regression
Han Yoohyun shows Han Yoojin, Bak Yerim, and Yoo Myeongwoo a Gate Stone he's carrying and explains its purpose as part of their novice Hunter training. He later offers said Gate Stone to Han Yoojin after the Gold Beak Maiya unexpectedly appears inside the low-grade mid-rank dungeon they're attacking; however, Han Yoojin turns him down once again, and Yoohyun presumably puts the Stone back in his inventory.

Trivia

 * The initial release of Webtoon Episode 5 depicted Gate Stones as long, faceted gray stones with a pair of glowing golden lines etched into their top facet. The episode was later updated to rectify this error.