I should have said that most of the balls are usually ascribed to the
"late neolithic" without any specific dates. So we might take the dates
to be 2,000 BC plus or minus, and so in the same period as the early stone
circles. A few are known to have knobs which have been worked with metal
so that the archaeologists see them as still being made in the iron age.
Most of the balls are about 70mm in diameter and made of various stones
from hard granite to relatively soft sandstone. There are three known
unfinished balls that suggest (as Robert Tragesser surmised) the stone
was shaped to a sphere before the protuberances were carved out. Most
of the known balls have 6 "knobs" - these being discs just marked out, or
prominent square cut knobs, or round projecting bosses. The decorative
ones mainly show spirals.
For what it is worth I note that when people nervously knead bits of bread
at mealtimes, they most often produce six sided cubes. I once asked a
class of ten-year olds to "decorate" soft clay balls using one basic design
repeated in some way if they wished. They produced a number of designs, but
mainly with 3,4 or 6 repeats. I showed them pictures of some of the
Scottish balls and they were then eager to make more complicated
arrangements.
Dick Tahta