A gem with no glass around it. It leaks Stormlight within days, but that permeability is what lets it recharge fabrials and Shardplate, and lets a Radiant breathe from it as though from a sphere.

Summary

A gem with no glass around it. It leaks Stormlight within days, but that permeability is what lets it recharge fabrials and Shardplate, and lets a Radiant breathe from it as though from a sphere.