Adamantite Weightstone

An Adamantite Weightstone is a temporary enhancement for blunt weapons, like a mace or a staff. It improves the weapon's damage and chance to critically strike.

Source
This item is crafted by Blacksmiths with a skill level of 350.

are sold for by You must be  with Cenarion Expedition to purchase the recipe.

As of Wrath of the Lich King, this is the most powerful weightstone available; however blunt weapons above iLevel 165 are ineligible.