I think it's a floral tiara, there is a better picture at the first page of the thread. Alexandra got this tiara and a copy of Queen Victoria's Diamond Jubilee necklac from her husband to use as a parure, since Princess Marina left her jewels to her sons. I have also seen it worn with turquoises and instead of diamonds.
Speaking about diamond , I ve been lately trying to locate which pieces Queen Olga of Greece gave to her granddaghters and I believe that her diamond stars (she had 3 or 4) divided between Marina and another granddaughter. However , I haven't seen any pictures of Marina wearing diamond stars , only Princess Alexandra. Has someone seen some picture?