Bloodstones

Bloodstones are ruby red gems containing a great deal of power that can only be awakened by "feeding" them blood. According to the studies recorded in Belamoore's Research Journal, these stones are neither living nor dead. Belamoore also felt the presence of a deep, hidden, hungry power, yearning for release. They are terrible weapons.

Background
Bloodstone is sometimes used to assist in demonic summoning. They are often inserted into pendants and amulets, though they can be depleted. The warlock adventurer used a Bloodstone Choker at a summoning circle to summon a voidwalker.

The most famous ones are the Bloodstone Artifacts, powerful relics that belonged to Lady Sylvanas Windrunner and were held by the Forsaken in a secure vault in the Undercity. They were stolen by a group of four traitors led by Kegan Darkmar. The thieves brought them to Dalaran, where the wizards would quarantine them in the Lordamere Internment Camp. The Syndicate had Bloodstone Amulets that they used to summon demons.

Kira Iresoul, a warlock of the Black Harvest, is also in possession of a bloodstone. Kira's Bloodstone was fed with the blood of hundreds of demons and ultimately used to submit the powerful Eredar Twins, Lady Sacrolash and Grand Warlock Alythess, for the Black Harvest's purposes.

Related items

 * Infernal Bloodstone Shard
 * Bloodstone Amulet
 * Befouled Bloodstone Orb

Speculation

 * The origin of the bloodstones is unknown. As described, the bloodstone artifacts seemed to be connected to the Old Gods, as their powers were alike to the Old Gods' yearning for release. They may be physical by-products of such a proposed Old God, akin to Yogg-Saron's blood (better known as saronite).
 * However their heavy association in quests with warlocks and demons implies a fel origin instead.
 * Bloodstones may be infused with all elemental forces.
 * The Demon Seed, a red crystal floating above a demonic altar of fire used by the Burning Blade to communicate with the demons of the Burning Legion, could be a bloodstone.