The Gem of Earth Power is an essence gem which grants a moderate bonus to Body. It can only be used by followers of the Way of the Closed Fist, and unlike many other such gems, it has no Way of the Open Palm equivalent.
It can be obtained by defeating the Forest Shadow in battle.
In-game description: "Tearing winds and the coursing waters test their strength, but the mountains endure. Wearers of this gem gain some of this strength, their bodies hardened by its magic."