"Sling-shots and shaman: The archaeological enigma of Pictish painted pebbles"
Article to be found in Culture24
Interesting bit on tools used.
"To reproduce the perfect circles seen on the Buckquoy pebble, two ‘tools’ were used: the shaft of a large seagull feather cut in half, and the hollow stalk of Wild Angelica, both of which were used with good precision to duplicate the original designs."