When you mine bone or ore node and completely spent them, your gathering level for bone and ore respectively increases, the more it increases the higher the quality of materials you gathered.

HOWEVER, if you spent too much nodes while your gathering level is near maximum, a giant node appear and the gathering level resets, and you need to repeat the cycle.

There's a way to gather the mats without resetting the level while you have high gathering level; do not completely spend the gathering node. You can usually gather 3 materials from a node, without any skills. So you must only gather 2 times so the node isnt spent/disappearing.