Oscar Ozdaslar ran after the thief after he snatched two rings and a bracelet out of a display case at Northern Diamond jewellers in Aberdeen.
The thief managed to escape out of the door and was chased around the corner and up the street by the businessman. But he managed to get away and disappeared from sight along with about £21,000 worth of stolen goods.
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Mr Ozdaslar said the thief who had come into the Union Street store and had asked to see a necklace on Friday.
He said: "I was shaken up but the police calmed me down. No-one is hurt and the guy will be caught."