But despite their historic significance, the purpose of Pictish painted pebbles is still unclear.
Now an attempt has been made to recreate them in Shetland.
Robbie Arthur, a stonemason, and Jenny Murray, of Shetland Museum and Archives, studied the existing examples to understand their decoration.
Mr Arthur observed that external gable walls containing the unlined, stone-built flues for open hearths often show a dark, blackish-brown stain, very similar to the colour of the spots on the painted pebbles. The distillation of vapour, from the burning of peat, occurs when the flue cools, producing a sticky deposit.
When a fire warms the chimney again, this tar dries to a dark and shiny substance resembling pitch. It was this substance that was used for the experiments.
In an attempt to replicate the original designs, various methods of application were tried. At first, a straw was used, and this replicated dots, S-scrolls and lines similar to those on the Scalloway pebbles used as examples.
Ms Murray said that whatever their function, painted pebbles were likely to have been highly valued by the people who carried them.
She said: "Their numbers in the archaeological record appears to have been limited, perhaps suggesting they were of revered significance, held by a certain few."
A new catalogue of painted pebbles records the location of these finds throughout Scotland, with single examples from as far south as Dumfries and Galloway and west to the Outer Hebrides. It also highlights predominant clusters of finds within the northeast regions of the Highlands and Islands.
Mr Murray said why they appeared more frequently in Caithness and the Northern Isles remained a mystery.