(CNN)Hundreds of elaborately carved stone balls dating from Prehistoric Scotland have been discovered over the centuries, but their purpose remains a mystery.
More than 500 of the stone balls have been found in total, most sized to fit neatly into the palm of the hand. They have been shaped so that a number of knobs protrude from the surface and some have beautiful, intricate patterns incised onto them.
So elaborate are the carvings that early archaeologists didn't believe it was possible for them to have been made using flint tools, so they dated them to a later period. But we know they were indeed carved using flint and date back to around 3,200 BC to 2,500 BC, a time when people in Scotland were leaving their lives as hunter gatherers and settling into life in farming communities.
What were they for?
Although no hard evidence exists to definitively determine their function, many have speculated as to the stones' purpose.
Some believe that they were part of a weighing system for primitive scales, but others argue that their weights vary too much for that to be practical. They might have been used to weigh down fishing nets, or as bearings to move bigger rocks, but then why would they be carved so elaborately?
Australian author Lynne Kelly has proposed that the stone balls served as "memory devices" that could have been used as mnemonic aids to the oral history of the times, much like Australian Aboriginal cultures used rock art and their surroundings.