It's been a while but you have to go to the cemetary (in Oakvale). Make sure you have a spade. Upon entering the graveyard, 2 men will be there talking. SNEAK around them QUIETLY (up around on the hilltop)and listen to them speak. (I think this is the only way to release the necklace). After that, you'll find the necklace buried on the beach (between the two upside down boats) in the sand.
If you're afraid of the cemetary, try digging for the neclace without going there. It might not work though.
"You don't know what you know until you know what you don't know".
'TeemByrd'