Awww... I'm sorry you lost your necklace. Did you try calling it? ( like "necklace, necklaceeeeeeee... where are youuu... come to mommy..." or, I know, I know !!!! why don't you offer a juicy and very hard to resist reward to the Elves of Jewerly...okok, am I joking?? ... not sure =s )
I like Jura's answer, try remembering everything you did the last time you saw it. It has to be somewhere (unless those elves are real...in that case, please do what I suggested in the first parentesis) and you will find it. Be positive about that... gooooood luck !!!!
^^