Article:
D&D 4th ed: Mood Crystals, of Death!

Soul Stones are not a new invention, they exist in many fantasy settings, such as Warhammer, Diablo and World of Warcraft. The general theme is a physical gem that can store the soul or life energy of a living being. The reason this idea is shared across the genre is because it has a lot of potential, and it is fun!

Soul Stones vary in appearance, the most common is a medium sized ruby red gem, either dull or gleaming and glossy, dependent on it's current charge.
A Soul Stone can originate as a result of forbidden magical rituals, or perhaps something more exotic, such as the crystalized blood of a slain god or primordial; perhaps the eternal tears of Tiamat?

The story of their origin can be as specific or generic as you choose.

The first indication that these are not ordinary gem stones is when they are picked up and examined by the unwary adventurer. With a sharp pain, the magic taps into the very life force of the victim, draining vitality as it fills itself with a warm, healthy glow, or in the alternative, filling the adventurer with stored life force, stitching wounds and banishing ailments.

A basic Soul Stone has one simple function; it transfers life force in the form of a healing surge.

When empty, the stone will attempt to drain one healing surge from any living thing that grasps it without some form of protection (such as a glove).

The stone makes a basic attack vs the victim's Fortitude defence and if it succeeds, the victim loses one healing surge.

The next living thing to grasp the stone will have the option of gaining a healing surge worth of hit points (or temporary hit points if this amount will put them over their maximum). Although the victim might have a lot of hit points in their healing surge, only the receivers own surge amount is gained, never the same amount that the victim lost when drained by the stone (even if the victim's surge value was lower than that of the receiver).