Now Queen Mary has converted a necklace of the Crown Jewels into a bandeau tiara, there may be some other options still around=
For example the diamond choker of this set could be mounted on a frame and would make a possible tiara, with some changes to make the base go in a straight line:
Queen Mary has worn several pieces of the magnificent Danish Antique Diamond Parure in recent weeks, which is composed of three centuries-old heirlooms inherited from The Netherlands and Sweden
royalwatcherblog.com