Some mysteries are better left unsolved. But if you want your mystery solved, you're going to have to do detective work. It may be an old rare that was given out during a single specific event that maybe no rare collector around today was involved in.
So a few key questions you're going to want answer are.
1) Who was the owner of the house that went IDOC (if you don't know you will probably have less than 1% of ever finding out)
2) When was this player last seen?
3) When was this player most active?
This will possibly help identify the age of the item. If the person didn't actively play (but left there account open), it could mean the items age could range from the time they actively played or earlier.
an example would be if the person played actively during 2005 and won the necklace as an event reward in 2002, you would know the item is possibly/at least 3yrs old (because they actively played in 2005) and it has the possibility of being older. That would pretty much classify it as an event item of some type.
Try to dig up as much dirt about the previous owner of the necklace as you can manage. Once you begin un-tying the knot, you may even uncover the full story/history of the necklace.