Known as the Portland Tiara, it is estimated to be valued into six figures and was stolen from the Portland Collection in a raid on the Welbeck Estate.
Burglars used power tools to break in between 9.45-10pm on November 20 and stole the tiara and a diamond brooch from an armoured glass display case.
A spokesperson for the Welbeck Estate said: “Security services missed the thieves by 90 seconds and the police arrived on the scene two minutes later.”
Nottinghamshire Police are investigating and appealing for information about a silver Audi S5 suspected to have been involved in the robbery.
There are fears the tiara and brooch will be broken down and the gems sold on separately.
The 6th Duke of Portland commissioned Cartier to create the tiara for his wife, Winifred, Duchess of Portland, and she wore it to the 1902 coronation of King Edward VII.
The spokesperson added: “The tiara is part of the historic Portland Collection. It has been on display to the public at Welbeck since March 2016 and was previously loaned to the V&A Museum for its tiara exhibition in 2002.”
The centre-piece of the tiara is the Portland diamond, which dates from the 19th century. It is flanked by two diamond drops and other pendant diamonds, all set in gold and silver.
Richard Edgcumbe, curator of jewellery at the V&A, said: “The Portland Tiara is one of the great historic tiaras of Great Britain. Since its creation by Cartier in 1902, using diamonds from the historic collections of the Dukes of Portland, it has been recognised as a jewel of supreme importance, a superb design magnificently executed.”
Police ask anyone with information to contact police on 101, quoting incident number 856 of November 20 or contact Crimestoppers anonymously on 0800 555 111.