ok pal you say you've got to oakvale, right? and found the thieves right?
good go around behind them (the wall that is) and LISTEN IN they will say where the necklace is hidden!!!
( if you can't wait it is in between the two boats on the western beach by the jetti dig between them and then take it to lady grey
hope this will help you!
if i think not, am i not? i think not, don't you think?