Adamantite Sharpening Stone

An Adamantite Sharpening Stone is a temporary enhancement for sharp weapons, like a sword or axe. 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 sharpening stone available; however sharp weapons above iLevel 165 are ineligible.