The Gem of Unyielding Strength is an essence gem which grants a very large bonus to Body at the expense of a moderate penalty to each of the other two ability scores. It is a counterpart of the Gem of Inner Genius (which does the same for Mind) and the Gem of the Infinite Spirit (which does the same for Spirit).
It can be found in the audience chamber of the Imperial Palace in Chapter 7.
In-game description: "This gem makes its wielder capable of great physical feats that are normally only possible with years of training. The sudden improvement to the user's physical abilities disrupts inner balance, however, and so the gem also siphons essence from its wielder's spiritual and mental capabilities, weakening those aspects to fuel the wearer's unyielding body."