Healthstones (pre 4.0.1)

A Healthstone, or "cookie", is a conjured item created by a warlock using the Create Healthstone spell and a Soul Shard; it can be traded to another player and heals the player for a certain amount of damage when consumed. As a conjured item, healthstones are lost when a player logs out for more than 15 minutes. Healthstones are especially useful as they are on a separate cooldown from healing potions and Mana Gems.

Stacking
Warlocks can spend 1 or 2 talent points in the demonology tree to increase the healing power by 10% or 20% respectively. Before patch 3.0.2, Stones created by Warlocks of zero, one, or two points invested in Improved Healthstone were all considered different items, even though they looked the same. For example, it was possible to carry three different types of Major Healthstones:, Improved 1 , and  Improved 2. However, the Echoes of Doom Patch nerfed this ability, and now Healthstones in General are unique items, no matter the Ranks of Improved Healthstone the Warlock had.