Just going to leave this here:

Call me crazy, or just very interested in information dealing with certain NPCs or even mythology itself, or maybe the Dev's just got creative with this but!!

The following characters: Berched, Merlin, Dowra, and Young Tarlach (which you'll have to notice their NPC portraits for this), all tend to have some sort of blue gemstone as a necklace or earring.

What even is this? Because I'm dying to know what it is or if it has anything to do with magic or light, I have no clue.

I've assumed it's just my imagination, could just be a fancy piece of Ladeca. But it's neat and I wondered if they were related. c:

A "fancy piece of Ladeca" is the most likely thing based off our current knowledge so you might have already answered your own question, lol. Another possibility is just that some character designers just wanted to put a glowy blue thingy somewhere on the character and never actually considered what it would really be. Unless one of the keyword discussions actually mentions what it is though, I don't think we'll ever know for sure.

Yeah, I suppose so.

I was just really hoping there was a reason behind it, some interesting story added that's all! c:

"Fashionogi", latest trend in the NPC world, maybe.

Millia has one as well, albeit a bit different. Disincluding Dowra, my guess is that simple dnd-esque mage jewelry. Dowra's seems to be haphazardly stuck on, but the rest fall in line with it.

Couldn't find any other NPCs with them though.