How do I keep the clasp on a necklace from rotating to the front?

Can you unobtrusively fasten the jewelry piece to the chain so that it doesn’t slide on the chain at all? Failing that, which may not be desirable for several reasons, perhaps you can add unobtrusive stops in the chain that prevent the jewelry piece from moving “too much”.

I think the clasp tends to move toward the lowest center of gravity – which the jewelry piece itself also wants to occupy, of course – so whenever the chain slides backwards or forwards, the clasp is going to be biased towards “wherever is closer to the center of the Earth”. And so it goes, and keeps going.

My point is that if you can get the jewelry piece to always or nearly always occupy the favored spot in the center of the chain, then it’s a lot less likely for the clasp to move, because it has to fully overcome the inertia of the jewelry piece at the center.

Is the chain very light relative to the clasp? My guess is the clasp weighs more than a good portion of the chain so it falls down to the bottom next to the jewelry piece.
This also shows you are an active person.despite what you might think. :-)
To prove or disprove my theory I’d open the clasp and add a small length of fine wire that weights much less than the chain. You can get a roll of the stuff at Lowe’s for ~$4.
Every household should have one as it is useful for other projects.
Make a small loop on both ends and hook the clasp to them. That will change the mass distribution and leave the lightest portion at the back of your neck. Don’t worry about how it looks. This is for diagnostic purposes.
Unless the problem is you unconsciously playing with the chain – or walking with a limp – that should work.
If it does you can change the clasp to something smaller and lighter or size it such that it fits inside the tubular piece out of sight.

Sad news from Jewelry Secrets.

Ok, so gravity defying metaphysics it is!

While I’m certain that @LuckyGuy‘s experimental procedure – and replacing either the hasp or the chain itself would help to fix the problem, I think that – properly understood and implemented – my suggestion would work just fine.

Here’s a different way to do it: Take the thinnest thread that you can and tie the piece of jewelry into the center of the chain. Alternatively, tie the thread in small knots onto the chain on both sides of the piece of jewelry. That will make it act as a relatively unmoving pendant and keep it centered where you want it.

@CWOTUS I like your idea. It is easy to implement.

When the clasp comes to the front you pick it up
and kiss it and then pull it to the back….it stands
for good luck when that happens! That is what I
was taught.