Moony Jewel is a CE with an unique effect that increases the servant’s resistance to charm by 80% or up to 100% when max Limit Break. This is a practically useful CE when facing against enemies that have charm skill such as Stheno and Euryale. However, outside of that, this CE has zero use at all except using it to farm for more dangos.
As usually only female servants have the ability to charm the opposite gender, therefore this CE is suitable to be equipped by male servants. Besides that, as the CE also provides HP stats, it is recommended to use it on Berserkers to help increase their survivability.