Police will be closely watching the movements of a British tourist amid fears he might have swallowed a rare $200,000 diamond he’s accused of pinching.

A Cairns jeweller plans to give his $200,000 stolen diamond a thorough clean if it’s discovered the alleged thief swallowed the precious stone.

“We’ve got ultrasonic cleaners here for that type of thing,” The Diamond Gallery owner Keith Bird told AAP with a grin.

He alleges his rare pink Argyle diamond, usually kept in a safe, was snatched from his hand as he held it out for a customer to inspect.

It’s alleged the thief used a mountain bike to make a getaway.

Police have taken a 29-year-old English backpacker into custody, but there is no sign of the jewel.

Mr Bird has praised police for their efforts, and is hopeful the diamond will be returned because he already has a potential buyer.

“We had the best night’s sleep that you can imagine last night,” he told AAP.

Cairns Detective Acting Inspector Kevin Goan says X-rays haven’t shown if the man is still in possession of the stone; however, police haven’t ruled out the possibility he ingested it.

Asked whether police were keeping a close eye on his movements, Insp Goan said: “Most definitely”.

The alleged thief was arrested at Tullamarine Airport in Melbourne on Thursday night, where he planned to board a plane to Auckland before heading to the UK.

The man is expected to arrive in Cairns, from Victoria, about 10pm (AEST) on Friday and will face court on Saturday, when he will be formally charged.

In an unrelated case in November, police arrested alleged robber David Watts, who was accused of swallowing two diamond rings during a jewellery-store heist in Brisbane.

A $27,000 ring was put up for auction after being recovered by police, while the other, worth $35,000, was lost after police threw out a bag of waste without searching it while the suspect was in custody.