Mystery Upgrade Stone I can increase or decrease an item's stats by up to +3 or as low as -3 per roll. If an item has a Misc effect and the item is not crafted, Mystery Stones can increase or decrease it by +/-1 per roll. Mystery stones can not affect the Misc of crafted items.
You must complete all of Krunk's quests and talk to him after reaching Mining level 20 in order to receive the token, which falls on the ground. He says 33 at first but it's actually 20.