Wondrous Item, very rare
This crystal has four charges. You can expend 1 charge to spend 1 hour magically transferring some of the protective power of this crystal into a piece of armor or clothing. After doing so, the armor or clothing becomes magical if it was not already, and it grants you a +1 bonus to your AC while you wear it. If the armor or clothing already provided a bonus to your AC, then that bonus increases by +1 (up to a maximum of +3).
The bonus from an Aegis Crystal does not stack, and thus a creature can benefit from only one piece of armor or clothing enhanced by an Aegis Crystal at any given time. Once a the final charge of this crystal is used, the crystal becomes nonmagical.