1976 government papers
By Dominic Casciani
It is said to curse any man who wears it, but the Koh-i-Noor diamond remains an object of great beauty.
Queen Mother's funeral: Crown carried the Koh-i-Noor
However, in the 1970s, the British government became exercised over whether the UK's ownership of the ancient and famous stone was entirely legal after Pakistan, out of the blue, requested its return.
Documents released to the National Archives show how that request was taken seriously by the Callaghan government of 1976.
Teams of civil servants were asked to make sure that Britain could say that its ownership of the 105-carat diamond was fair - although one official said it did not matter whether it was legally in the UK.
According to the archives, the then Pakistan prime minister Zulfiqar Ali Bhutto wrote to Jim Callaghan asking politely for the diamond's return.
Mr Bhutto said there was a "sense of cultural deprivation or historical disinheritance" caused by Britain hanging on to precious objects collected around the British Empire.
Callaghan: Conscientiously looked into ownership
"There is one object of great and vital historical interest, the Koh-i-Noor diamond, which was taken to London in 1849 from no other place than Lahore, Pakistan's cultural capital," said Mr Bhutto.
"The object is financially inconsequential for Britain by contemporary standards but its long history lends to it immense sentimental value for Pakistan.
"Its return to Pakistan would be a convincing demonstration of the spirit that moved Britain voluntarily to shed its imperial encumbrances and lead the process of decolonisation."
There were only two slight problems with Pakistan's request.
Firstly, it was not the only country with a claim to the diamond.
Over the course of 500 years the legendary diamond had passed from India to Persia - and then on to Afghanistan before returning to India and the treasury of Lahore.
OURS NOT YOURS
The stark facts are these: we have the Koh-i-Noor diamond, whether or not our possession of it is legally justified
Foreign Office official CO Hunt
The British took the diamond as part of a peace treaty as they built the empire in India and sent the stone to London.
The second problem? The diamond had been placed in the Crown Jewels, where it remains to this day.
When the diamond came to London during Queen Victoria's reign, it was recut, reducing its size but making it more brilliant. It was placed in the Imperial Crown and in turn passed to that worn by Queen Elizabeth, the Queen Mother at the 1937 coronation of her husband, George VI.
Told in a memo that the stone was in the Tower of London, Mr Callaghan scribbled in the margin: "I thought it was - very awkward!"
But the papers reveal the Callaghan government took the request seriously and sent officials to investigate its ownership - albeit in as low-key a way as possible.
Documents in the Foreign Office library were searched, government lawyers instructed and soundings taken from embassies.
Diplomats in the countries likely to counter Pakistan's claim were cabled and told to do as much as possible to kill the story.
The prime minister consulted the Queen. However, her exact reply is not known as a note from Martin Charteris, her private secretary, to the PM remains secret.
But one Foreign Official, CO Hunt, was rather annoyed at all the time and energy being expended on the issue.
"I can work up no enthusiasm whatsoever about the historical legal minutiae... nor do I think we need to," he said.
"The stark facts are these: we have the Koh-i-Noor diamond, whether or not our possession of it is legally justified. We have made it clear that we are keeping the diamond, adducing the best arguments to support our contention."
In other words - tough luck. The prime minister replied to Mr Bhutto with a polite "No".