Princess Isabella of Denmark is marking a major royal milestone for her 18th birthday. The second child of King Frederik and Queen Mary celebrated her big day on Monday, April 21—and her first ...

The tiara has single, rose-cut and brilliant-cut diamonds and three fluted turquoise plumes set with diamond stems. The auction house had estimated a sale price of between £250,000 and £350,000.

The diamond-encrusted tiara - expected to fetch about £400,000 - reaches nearly £890,000.