These are lab-created GAGG (Gadolinium Aluminum Gallium Garnet) facet rough with pronounced phosphorescence (glow in the dark). Each piece is marked with its weight and you select which one to purchase.
Note that, as a particularly defective crystal, some of these pieces will have a few bubbles. The bubbles tend to correlate with the highest afterglow.
This is "Long glow" or "high glow" GAGG--a type of defective GAGG which exhibits a glow-in-the-dark effect. They work pretty hard to avoid this happening since it makes the crystal useless for its intended purpose (detecting invisible radiation). However mistakes happen and sometimes a crystal does show this effect. Each one is a little different because the causes can vary, but this crystal shows a bright but short-lived initial glow which fades to a dimmer but long-lasting glow. It was visible after at least an hour of sitting on my bedside table in the dark.
Cerium GAGG is brand new as far as gems go--it only entered industrial usage in 2014, and we just introduced it to the gem market last year. It is what we call a 'lumogarnet' - an exotic lab-created garnet with incredible fluorescence. We also call this 'Firefly Garnet'.
Like Ce:YAG and LuAG GAGG is used as a scintillator, a crystal grown for its insanely strong fluorescence. It is used to help detect invisible radiation by absorbing it and converting it to easily-detected visible light--gamma rays and x-rays will activate its glow in addition to UV and even normal blue light.
Properties-wise 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 material has very bright fluorescence, and like Ce:YAG and LuAG it is used as a scintillator to help detect invisible radiation.
