The Crystal Shield is a direct upgrade to the starter shield.
It can sustain a greater number of attacks than the basic shield before the wanderer's guard is broken. It also glows in the dark, similar to the Crystal Skull and Crystal Lance.
The shield's design is reminiscent of the enemies that ambush you in the tech-realm elevator section of Floor 4.
The Crystal Shield can be found on Floor 20. (More information is needed here. Feel free to edit if you know how to get to it!)