I was thinking more about backing one of those faux gems with fluorescent tape, or spritzing it with clear fluorescence. It would glow whenever it was charged by actual lights, but still fade over time.
But now I just want to cast light spells on it. See what you did?
I did that with my MWE horns once. I think the important thing is to explain in-game that the reason they are glowing was because you had coated them with glowing mushrooms or something -- in other words, we need a non-magical explanation.
"It's all the fashion to do this in my community."