The Gem of the Infinite Spirit is an essence gem which grants a very large bonus to Spirit at the expense of a moderate penalty to each of the other two ability scores. It is a counterpart of the Gem of Unyielding Strength (which does the same for Body) and the Gem of Inner Genius (which does the same for Mind).
It can be found in the Inner Courtyard at Dirge.
In-game description: "A powerful spirit usually takes decades to attain, but this gem offers ambitious seekers a short cut to that lofty goal. The gem empowers its wearer's spiritual energy by imbalancing the user's physical and mental abilities."