The Gem of Evil Thought is an essence gem which grants bonuses to Mind and Intuition. It is exclusive to followers of the Way of the Closed Fist, but another gem, the Gem of Gentle Mind, can provide the same bonuses to a follower of the Way of the Open Palm.
It is found at random.
In-game description: "The evil energies that resonate in this gem speak of its previous owners, dark and evil men and women driven beyond the boundaries that society imposes on them. This is a gem of murders and thieves. Their thoughts and skills are reflected through this gem and passed on to its user."