The Gem of Pure Flame is an essence gem which grants moderate bonuses to Spirit and Mind. It can only be used by followers of the Way of the Open Palm, but the Gem of Black Flame and Burning Essence can provide the same effect for other characters.
It can be obtained by completing Fading Moon's Bounty.
In-game description: "Flame can destroy, but it can also purify. Burning with inner fire and zeal, users of this gem find themselves more focused in battle and better able to tap their spirit's reserves."