This is one of the newest tiaras in the British Royal Family's collection. The late Elizabeth II comissioned it in 1973 from the jeweller Garrard.
It was made from diamonds from a tiara that was dismantled (the Nizam of Hydrabad tiara), along with 96 rubies given to Elizabeth as a wedding gift from the people of Burma. The rubies were set into a series of roses.
Elizabeth II wore it often, but in my opinion it didn't really suit her. However recently Queen Camilla wore it for the state banquet to honour the President of Korea and she looked magnificent in it.