It's A Slime

The crystals were surprisingly light for their size to the point I was afraid they would break at the softest touch.

But Jack showed me that he could toss it at the wall and it would still remain intact relaxed me somewhat.

Then again, the fact that they were swinging their pickaxes at the crystals with their full strength should have clued me in on that.

Harper stored the crystals into the backpack he was carrying along and we proceeded onwards deeper into the Dungeon.

At this point, I couldn't help but ask, "Judging by the number of people we saw at the entrance… Isn't it weird haven't we seen any other Dungeoneers?"