The famous Portland Tiara has been stolen from The Welbeck Estate in neighbouring Nottinghamshire last week.
Burglars used power tools to break into the Portland Collection Gallery on the estate in Worksop between 9.45pm and 10pm on Tuesday, November 20.
They stole the tiara, a national treasure seen by countless people, and a diamond brooch from an armoured glass display case while the alarms were sounding.
Nottinghamshire Police are investigating and appealing for information about a silver Audi S5 suspected to have been involved in the offence.
This vehicle was found abandoned and burnt out in Cross Lane, Blidworth, about half an hour after the incident, Nottinghamshire Police said.
Detective Inspector Neil Humphris at Nottinghamshire Police said: “We’re pursuing a number of lines of enquiry but we believe there are people out there who may have crucial information that could help with our investigation.”
The 6th Duke of Portland commissioned Cartier to create the Portland Tiara for his wife, Winifred, Duchess of Portland.
She wore it to the 1902 coronation of King Edward VII and the Duchess was one of four pall-bearers at Queen Alexandra’s anointing.
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.
The brooch is composed of diamond clusters that previously stood at the apex of the tiara.
‘Missed the thieves by 90 seconds’
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.
“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.”
Curator of Jewellery at the Victoria and Albert Museum in London, Richard Edgcumbe, 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.”
Anyone with information should contact police on 101 quoting incident 856 of November 20.
It can also be reported through Crimestoppers anonymously on 0800 555 111.