The Mossy Stone is a possibly-guaranteed artifact armor. The author of this page found his instance inside of the vault, although it could have been part of the randomly generated loot.

The armor itself isn't that interesting, as it doesn't contain any special enchantments and only has fire/cold resistance. However, the armor reduces incoming damage by an incredible amount (8 points of damage and 26%), and it also increasing dodging by a significant number. These two properties make this piece of armor quite valuable as you progress into the midgame, since at that point elemental resistances should be quite high (or maxed) whereas physical damage reduction becomes very necessary.