An unusual piece of bone was discovered during the search at a property in South Australia, which forensic archaeologists believe could belong to a human child.

“It was obvious it had unfused growth plates, meaning it was from an immature person,” Professor Maciej Henneberg told 9News.

Henneberg said he was 90 per cent certain the fragment could form part of a small human pelvis.

Ratcliffe was aged 11 and Gordon was aged just four when they were abducted from a football game in Adelaide over 50 years ago.

A private team investigating their disappearance began searching at a property in the state’s mid-north which once belonged to now-deceased Stanley Arthur Hart.

Hart was once linked to the disappearance as a suspect but he was later ruled out by police. 

“It jumped out at us, [it is] appeared on-site to be glass,” said private investigator Bryan Littley.

Samples will now undergo forensic analysis

The team is legally obligated to turn it over to police if it is human bone.

“It’s as simple as, if your girls went missing, how would you feel?” Littley added.

“We are aware that our efforts raise hope.

“And that’s why we’re doing it.”

No one has ever been charged over Ratcliffe and Gordon’s disappearance.

And despite a $1 million reward announced in 2014, the girls have never been found.

SA Police said the disappearance of Ratcliffe and Gordon remains an “active and ongoing investigation”.