These are "high glow" glow-in-the-dark GAGG garnets! While all GAGG phosphoresce a bit these are top grade material that glow the brightest and longest of any we have.
Cerium GAGG is a material which has recently risen to prominence in industry but never been used as a gem until now. It is what we call a 'lumogarnet' - a lab created garnet with incredible fluorescence. It glows a bright yellow, slightly greener and brighter than already-bright Ce:YAG. This material is like an upgraded version of the classic GGG crossed with an aluminum garnet (like YAG and LuAG) to give it excellent hardness (8) while maintaining a high RI (1.9) and generally excellent gem properties.
This is our highest grade of GAGG, and it GLOWS IN THE DARK! That's right, I'm so excited about it I wrote it in all caps! They work very hard to make sure it does not glow in the dark since it lowers the temporal resolution of the system (you want to know where an x-ray is now, not where it was an hour ago!) but sometimes they make mistakes, and a small subset have a stronger glow. On top of all that it has a greener color than any of our other GAGGs.