A "brazen" thief who snatched a five-carat diamond worth more than $300,000 from a Sydney jewellery shop is being urged to hand himself in.
The man visited the flagship Cerrone Jewellers store at Martin Place on Monday, before returning on Tuesday and asking to see the stone.
He then switched the uncut diamond with a replica of lesser value and left the luxury store.
"It's fairly brazen," NSW Police Inspector John Bamford told reporters in Sydney on Friday.
"It's something that we wouldn't normally see."
Police have released CCTV of a man they would like to speak to over the incident, describing him as being of Asian appearance with a slim build and short black hair.
"The best thing that the man could do would be to hand himself in to Day Street Police Station," Inspector Bamford said.
He wouldn't comment on how the stone was swapped, nor speculate on whether the man may have been a professional or an opportunistic thief.
Shop manager Alison Kotevski told AAP staff were doing everything they could to retrieve the precious stone and catch the criminal.
The man visited the same store on Monday and asked to see the diamond before returning to pull off his suspected switch.