"carved stone balls" or if you want to be really technical, "petrospheres.") Imagine grinding a rock into a roughly spherical shape, then carving semi-circles out of its side.
These smoothed semi-circles are called "knobs," and were typically decorated with either straight cross-hatching, spirals, or concentric circles. Sometimes quite elaborately, and other times in a very plain style. Most of them have six "knobs," which forms one on each face if it were a cube. But they have been found with other numbers, with the record so far being a carved stone ball featuring 160 knobs.
The carved stone balls date from the Neolithic era, and are found mainly in northeast Scotland, which puts them within the territory and time span of the ever-mysterious Picts. Most of them are small, about 2.75 inches in diameter, or approximately the size of a tennis ball. There are 387 known examples, but likely many more have been found and discarded over the centuries, because they were broken, too plain to interest collectors, or found by someone who didn't realize what it was.
What would possess the Neolithic Picts to carve a ball out of stone, then carve smaller balls on its side? A number of theories have been proposed, most of them at least somewhat plausible. Many people have observed that the shape of the knobs makes it easy to tie a leather thong around them, which would make the carved stone balls useful as a swung weapon, or as a weight tied to fishing nets, or as a tool used in scraping hides clean to be dried for leather.
Other speculation includes such divergent theories as a divining tool, or as a symbol of authority and a speaking stone (similar to the conch shell in Lord of the Flies).
A large portion of the known carved stone balls have been carved to an exacting size, within a millimeter of one another. Although their weights vary widely (because they are carved out of different substances - likely whatever the carver had at hand), their almost uniform size may be a clue as to their purpose.
Or perhaps we are just uncovering the popular hobby of the day, a Neolithic version of the amazing Dorodango "marbles" that Japanese school children fashion out of mud and persistence.