I have a necklace that has one stone as the pendant. The chain on it keeps turning around my neck until the clasp stops at the front of my neck by the stone. How can I stop this from happening?
If you attach the pendant to the chain of the necklace, so that the chain (and thus the clasp) can't rotate without the pendant moving with it, the weight of the pendant will keep it hanging at the front, and the clasp at the back. Depending on how well you can hide it, a loop of metal our dab of hot glue should work.
Depending on whether this a necklace you use all the time, or just on special occasion an alternative solution could be to use some tape (or blank band aid) to fasten the necklace to the back of your neck. This of course only works if you either have long hair, or have other means to hide the band aid.
If you use it all the time, you are better of with somehow fastening the pendant to the necklace as suggested in other answer.