Is this totally stupid of me to do

You should ask for it back ASAP! Send him a text or call.

As long as you manage to keep it to strictly business, you should ask him for it. Don't let it turn into a lengthy and awkward conversation. It is an important piece of jewelry for you, you are all rights of asking for it. I would recommend you to ask for it asap, otherwise he could get rid of it, or give it away or something and you don't want that to happen to your necklace.

you could text him saying that you will pass by his house later in the day or the next day to pick it up to please make it ready. You knock say hi, get your stuff, tell him you hope he is doing OK and bye. turn around and start walking.

Why write him? Why not show up at his place and ask about it? If he can't find it right then and their say it's OK but just keep a lookout for it and he should return it. Guys don't care about jewelry.

It's fine to ask for it back, he shouldn't care one bit. But yeah if he hasn't spoken about it then hopefully he still knows where it is.
However on the chemistry side of me : White gold isn't real, its probably like 3 karat gold and 21 karat silver. (mostly silver)

You should have asked for it back sooner.

Hopefully he still has it.

Hopefully he did not pawn it, sell it or give it to another girl. He is a jerk to not of voluntarily given it back by now. Just call and tell him you are coming by to get it. Keep it simple.

no. It's important to you, he should understand and return your property.