Diamond appraisal wrong!!!

Had diamond appraised at a local well known, well established jewelry store. Received proper appraisal information on the diamond for insurance purposes. After long consideration thought about selling the stone. Brought it to a NYC jeweler who buys diamonds. Informed me that the stone was Mossisanite!!! I cannot believe it!! I have written professional information identifying every inch and detail of this stone. Who can I trust??