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[Ata'mal Crystal]?
The Ata'mal crystal was an artifact of naaru origin bestowed to the eredar of Argus.[1] Now shattered into seven shards, which have become lost over the eons, the shards are seen as both symbols of holy Light by the draenei - eredar who escaped the corruption of the Burning Legion - and as potential weapons of immense power by both the Legion and the servitors of the Betrayer, Illidan Stormrage.
Origin
The Ata'mal crystal was a sacred artifact of the eredar, accessible only by their three leaders - Archimonde, Kil'jaeden and Velen. Then the Dark Titan, Sargeras the Destroyer, came to Argus and offered the eredar unlimited power and worlds to conquer in exchange for their loyalty. Archimonde and Kil'jaeden readily agreed, but Velen - who possessed the gift of Sight that the others lacked - saw the truth behind Sargeras' offer, and how he intended to lay waste to the known universe.
Praying for guidance, Velen's pleas were answered by the naaru known as K'ure. K'ure instructed Velen to take the crystal and rally anyone willing to listen to him, and travel to Argus' tallest mountain on the longest day of the year. There, Velen's prayers would be answered. As Velen and his followers awaited a sign from the naaru, Archimonde and Kil'jaeden had caught wind of Velen's plan and unleashed their man'ari slaves to attack. Raising the crystal into the air in desperation, Velen watched as the crystal was struck by a beam of light, and suddenly shattered in his hand. The crystal created a barrier of light that prevented the man'ari from reaching them. At that moment, K'ure provided their escape from Argus - the Genedar, a gigantic diamond-vessel that would become known by the orcs as Oshu'gun, or "Mountain of Spirits".
The Shards of the Crystal
The Ata'mal crystal was sundered into seven shards during Velen's escape, in the colors of light as seen when refracted through a prism - red, orange, yellow, green, blue, indigo and violet. All of them are named, but only five have confirmed colors. The colors of the remaining two are unknown; orange and indigo are the only colors left, but which crystal has which color is unconfirmed.
- Heart of Fury (Red)
- Brilliant Star (Yellow)
[Leafshadow] (Green)- Spirit's Song (Violet)
[The Eye of the Storm] (Blue)- Fortune's Smile
- Shield of the Naaru
Fates of the Crystal Shards
When Velen was captured and released by the Frostwolf chieftain Durotan, the Heart of Fury and the Brilliant Star were taken from him and utilized against the draenei. The Brilliant Star is believed to still be in the hands of the fel orcs.
Akama, leader of the Ashtongue Deathsworn who (supposedly) serves Illidan, sends players on a quest to retrieve the Heart of Fury from the Shadowmoon fel orcs at the Ata'mal Terrace in Shadowmoon Valley. Upon retrieving it, Akama then places it into the 
[Medallion of Karabor] and instructs the player to take it to the naaru A'dal in Shattrath City. After a continued quest chain, the Medallion is returned to the player, allowing him/her to enter the Black Temple.
Spirit's Song was last held by Velen and later lost in Outland; it was found in the hands of a Dreadlord named Culuthas in the Ruins of Farahlon in Netherstorm. An eredar named Socrethar is seen communicating via a Legion communicator, demanding the crystal from Culuthas. Upon reclaiming the crystal and delivering it to A'dal, the player will then be sent to create the key to the Arcatraz, the most heavily protected of the satellites of Tempest Keep. This is odd, as Spirit's Song is the most powerful shard, and is intelligent to the point where it refused to leave Velen's possession.
Leafshadow was held by Restalaan, long-time friend of Velen and captain of the guard in the city of Telmor in Terokkar Forest. This crystal cloaked the city, protecting it from ogres and their gronn masters. Durotan and Orgrim learned of its existence during a childhood trip to the city. It was utilized by Durotan - who remembered the necessary words to lower the shielding illusion - to reveal Telmor to his forces, and the orcs proceeded to sack the city. Once in the Horde's possession it was used against the draenei to hide the orcs' assault on Shattrath City until the last possible second. Its current location is unknown.
Illidari demons and their Broken slaves are currently trying to dig up another unknown shard in the Ruins of Sha'naar in Hellfire Peninsula. It is possible that the relic retrieved by Alliance players in the quest Sha'naar Relics is the Ata'mal crystal in question.
[The Eye of the Storm] was originally planned to be located by Kanrethad and Jubeka in the Black Temple and then delivered to Wrathion in Pandaria by the player character.[2] These plans never made it live, and so the Eye of the Storm has yet to appear in-game.
The other crystals' locations and purposes are unknown, but purported to still be in the possession of Velen and the draenei.