Rio Grande has been great to deal with for a variety of items/tools etc. Their non-pre-mounted stones and cabuchons (spelled wrong probably) are nice if you can mount/set yourself, which is not hard to do if you create a stepped hole in the thumb stud of the right size(s).
If you get pre-mounted and aren't a jeweler (I'm not), you also ought to get the "step drill" bit they offer to drill the hole to set to mounted gem in....I have always bought the premounted in 2.25 mm and only need the one size step drill. The premounted are real nice, and not bad prices at all (it has been a couple -three years since I last bought gems).
They have white or yellow gold in, I think, in a couple of different carats.
If you call Signity directly, talk with them. They give good advice.
A number of years ago, Signity attended some of the knife shows so they are well aware of the knifemaker market....