Thundering Skyflare Diamond

Thundering Skyflare Diamonds are meta gems that have a 100% chance on melee/ranged auto attack to grant 480 haste to the user for six seconds, with a forty five second cooldown.

This item is crafted with Jewelcrafting (420); taught by Design: Thundering Skyflare Diamond, a world drop.

While over continuous use this gem effectively grants 64 haste rating (480 rating * active 13% of the time), the fact that this triggers on the first auto attack of the fight means that the some portion of the 39 second 'down time' (between the end of one use and the beginning of the next) would have been lost in switching between mobs anyway.