Arcanum of Focus

The Arcanum of Focus is a enchanted gem that adds a spell power bonus effect to either a head or leg item. This effect will not stack with other enchantments on the item.

Source
This item was a quest reward from the quest Libram of Focus (quest) which started with Lorekeeper Lydros in Dire Maul. To complete the quest, you would turn in the following items:
 * 1x Libram of Focus
 * 1x Pristine Black Diamond
 * 4x Large Brilliant Shard
 * 2x Skin of Shadow

As a quest objective

 * The Savage Guard - Arcanum of Focus