I recently lost a Cleopatra type 14K gold necklace and I had no proof of the item. I sketched a picture for a description and had weighed some of my mother's jewelry along with my own so I had the heavy weight of the piece and that's what they went by because I didn't have an appraisal or picture. I offered to have affidavits done by witnesses who saw and knew the necklace but they said it was not necessary...
However, I had only a $10,000.00 insurance policy on jewelry in case I lost a piece (which I did) but it only covered $2,500.00 per piece and the only thing they could go by was pretty much was the weight of the gold... without an appraisal, they paid me $2,200.00.
Are there any photos of your mom wearing the ring? Do you remember any jeweler who saw the ring?...If you saw something similar in a jewelry store, can you get a print out as to the value?...
Other then these things, you are at the mercy of the insurance company who has no proof as to the size, cut or clarity of the stone and that it was platinum. I'm sorry but one last thing... Did you search completely?... There are two occasions where my mother cried over losing her diamond wedding ring and a pearl necklace finding them where she hid them almost a year later. I also lost a Krugerand never to be found again but didn't have any insurance at that time and didn't even think to contact my homeowner's insurance company and that was two years ago... Good luck in getting some help and finding a picture if nothing else.