Grindstone Master/Hit That Vein

[pedrooferraz]

how do i get the achievement?

[Sstavix]

You tap the mining nodes in certain sections -usually caves. Using items that increase your chance of mining successfully will help you get it quicker.

[pedrooferraz]

which cave?

[Sstavix]

Any of them. Even the cave next to the starting village (den of devils, I think?) Will have them. Look for the rocks with crystals to appear in the background scenery. Up to four can appear at a time, but sometimes none appear. So you may have to travel back and forth between the cave and another location for them to show up.