I thought that bracelet must be a retcon, but turns out it was there the whole time
I was sure that Penny's magic bracelet was being retconned into existence, but then I went back and checked ZB1 and, sure enough, she's wearing it during her introduction in Chapter 100 (and E52 of the anime).
The colour is golden in the anime but an indigo blue in the front and back illustrations for Volume 16 of the tankobons and on the new kanzenban cover. The design does seem to change slightly depending on the image: sometimes it looks like it's engraved with squiggly writing or hieroglyphs, and other times it looks more like coral (appropriate for a water dragon's talisman). Based on the cover illustrations and how it appears in ZB2, I'm guessing the coral-like appearance is how it's supposed to look.
It makes me wonder whether Raiku planned this little piece of lore from the start. He clearly put a bit of thought into the design at the very least, despite not drawing attention to it in the original manga.