It was one of the quest only "Conqueror's" jewels. The other two are Conqueror's Longevity and Conqueror's Potency. Conqueror's Efficiency is a popular choice of vaaling, to obtain the corruption implicit 2% increased Reservation Efficiency of Skills.
3% reduced Mana Cost of Skills
4% increased Skill Effect Duration
4% increased Mana Reservation Efficiency of Skills
The stone may yet bleed.
A simple tool to price check your items in path of exile by "copy and paste". It is that simple!
Price in Leagues