I guess Etsy could be a good place to look. It has gotten annoying in the last few years, but sadly it's still the best place for a lot of people to sell their crafts.

She might enjoy a unique handmade piece by a silversmith there just as much or more as a high-end necklace, and unique hand-made designs can be cheaper than diamonds. Maybe you can even reach out to a silversmith and design something together.

I tried searching for #jewelry on mastodon.social, and @kosmimatis@handmade.social was the first thing to pop up. Of course you probably want to look around more, but their latest post is not necessarily a terrible starting point.

In short, I think it's possible to go about it looking for a silversmith rather than a jewellery store. What it lacks in expensive stones it makes up for by being potentially personal. :)