Khadgar's Gem of Health

Increases the hit points of the Hero by 300 when worn.

This flawless diamond, set in a platinum fitting surrounded by tiny amethysts, seems to flash with an inner light. When held by a character or dangled around the neck on a chain, the flickering from within gradually shifts to match its owner's heatbeat. The first of these special jewels was created by the arch-wizard Khadgarr to help keep him alive during his trials. Since then, others have duplicated its special magic.

Source
Kael, Lady Vashj and Illidan can find one in Gates of the Abyss, chapter 5 of the Frozen Throne alliance campaign, Curse of the Blood Elves.