A bright green glass spearhead believed to have been made by Aboriginal prisoners on Rottnest Island 100 years ago has been discovered by students during a university excursion.

Professor Len Collard from The University of Western Australia had taken the students to the island, known as Wadjemup, last week to learn about its Indigenous history as a prison for almost a century.

They were at one undisclosed site, believed to be on top of a hill, when one of the students uncovered a sparkling object.

Professor Collard said it was a rare glass spearhead that was at least 100 years old.

"It's not every day that you uncover an artefact of this significance," Professor Collard said.

"We have unearthed clear glass and ceramic spearheads before, but never a spectacular emerald green glass spearhead like this one."

Spearheads used to hunt quokkas

Professor Collard believed the spearheads were used by Aboriginal prisoners for trade, building relationships and hunting quokkas.

"We believe the prisoners would find a place on top of a hill overlooking the mainland where they would make spear tips from scrap pieces of glass," he said.

Quokkas are believed to have been hunted using spearheads. ( AAP: WWF Australia )

"This discovery is important because it helps us learn about our heritage and remember our past, which is important for today and future generations."

The spearhead has been re-buried on the island to respect the Aboriginal tradition of keeping artefacts found in their resting place.

Almost 4,000 men and boys from across Western Australia were imprisoned on Rottnest between 1838 and 1931.

Initially, only Aboriginal prisoners were detained there, but later non-Aboriginal prisoners were also incarcerated there.

At least 373 Aboriginal men are buried at one site, known as Tentland, which for a long time was the only camping ground for visitors on the island.

It is the largest deaths in custody site in Australia and the largest known burial ground of Aboriginal people.