Sarah Waldron of the Campaign Against the Arms Trade claimed that the event would welcome "some of the most authoritarian regimes in the world".
"The deals done here fuel death, injury, fear and repression - yet instead of banning it, the Government helps make it happen.
"We are here to resist it, to stand in solidarity with the people facing the deadly consequences of its weapons and do what we can to stop this obscene event continuing."
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Although the diamond was believed to be cut in the 19th Century, its history and how it arrived at the car boot sale is unknown.