The Museum has around 27 carved stone balls in its collection which provides a cross section of all the types. Eleven types have been identified according to their decoration and more information on the types is available.
Marshall's study has been used to group the balls into types. Her paper can be found at Marshall D 1976 "Carved Stone Balls", Proc Soc Antiq Scot (1976-7) 40 -72, and also provides an extensive bibliography.
Carved Stone Balls are petrospheres, usually round and rarely oval. They have from 3 to 160 protruding knobs on the surface. Their size is fairly uniform, they date from the late Neolithic to possibly the Iron Age and are mainly found in Scotland. They range from no ornamentation (apart from the knobs) to extensive and highly varied engravings.
 Age and distributionCarved Stone Balls are around 4000 years old, coming from the late Neolithic / Bronze Age.
Nearly all have been found in north-east Scotland, the majority in Aberdeenshire, the fertile land lying to the east of the Grampian Mountains. A similar distribution to that of Pictish symbols led to the early suggestion that Carved Stone Balls are Pictish artefacts. The core distribution also reflects that of the Recumbent stone circles. As objects they are very easy to transport and a few have been found on Iona, Skye, Harris, Uist, Lewis, Arran, Hawick, Wigtownshire and fifteen from Orkney. Outside Scotland examples have been found in Ireland at Ballymena, and in England at Durham, Cumbria, Lowick and Bridlington. The larger (90mm diameter) balls are all from Aberdeenshire, bar one from Newburgh in Fife.
In the late 1970s a total of 387 had been recorded. Of these, by far the greatest concentration (169) was found in Aberdeenshire. By 1983 the number had risen to 411.
 Archaeological contextMany of the balls have not had their discovery site recorded and most are found as a result of agricultural activity. Five were found at Skara Brae village and one at the Dunadd hillfort. The distribution of the balls is similar to that of mace-heads, which were both weapons and prestige objects used in ceremonial situations. The lack of context is likely to distort the interpretation. Random finds are only likely to have been picked up and entered a collection if they were aesthetically appealing. Damaged and plain balls were less likely to find a market than decorated examples so some more decorated examples might be fraudulent.
 Physical characteristics
 MaterialsMany are said to be made of "greenstone", but this is a general term for all varieties of dark, greenish igneous rocks, including diorites, serpentinite, and altered basalts. Forty-three are sandstone, including Old Red Sandstone, 26 greenstone and 12 quartzite. Nine were serpentinite and these had been carved. Some were made of gabbro, and a difficult material to carve. Round and oval natural shaped sandstones are sometimes found. Examples made from Hornblende gneiss and granitic gneiss were noted, both very difficult stone to work. Granitic rocks were also used and the famous Towie example may be serpentinised picrite. The highly ornamented examples were mainly made of sandstone or serpentine. A significant number[quantify] have not as yet been fully inspected or tested to ascertain their composition.
 Experimental archaeologyUsing authentic manufacturing techniques (pecking and grinding), full replicas have been made by a researcher at the University of Exeter. It was shown that they could be made using prehistoric technology with no recourse to the use of metal tools.
 Size, shape and knobsOf the 387 known carved stone balls, 375 are about 70 mm in diameter, but twelve are known with a diamter of 90 – 114 mm. Only 7 are oval. They are therefore about the size of tennis balls or oranges.
Nearly half have 6 knobs, 3 have 3 knobs, 43 have 4 knobs, 3 have 5 knobs, 18 have 7 knobs, 9 have 8 knobs, 3 have 9 knobs, 52 have between 10 and 55 knobs and finally 14 have between 70 and 160 knobs.
The Towie ball has some design similarities with the carvings on the Folkton "drums". These were found in a tumulus in England and are made of chalk with elaborate carvings, amongst which are distinct oculi or eyes. Concentric carved lines on stone balls appear to be stylised oculi. This ball also has a roughly triangular arrangement of three dots in an interspace between the knobs. This appears to be identical to the arrangement of dots found on the Parkhill siver chain terminal ring, found near Aberdeen, a Pictish artefact. It is possible that the dots represent a name, as some of the Pictish symbols at least are thought to represent personal names.
New Grange carvings in Ireland show strong similarities to those found on some balls. A continuous spiral is found on one and elements of chevrons, zig-zags and concentric triangles are also found, stimulating comparisons with petrosomatoglyph symbolism. Mostly the different knobs have different or sometimes no ornamentation. A 'golf-ball' variety of ornamentation is found on a few balls. The carving does not appear to have any practical purpose in general, however it has been suggested that one type, with very distinct knobs, was used for processing copper ores (see under 'Function'). Some of the bold triangles and criss-cross incisions seem to be more Iron Age in character than Neolithic or Bronze Age.
 Functionbolas. Their use as weapons was suggested by many researchers but in recent years this idea has fallen from favour.
One suggestion saw the balls as movable poises on a primitive weighing machine, following the logic of the remarkable uniformity in size shown by a good number of these carefully made objects. However, it has been shown that their weights vary so considerably that mathematically they could not be considered part of a system of weight measurement.
'Sink stones' found in Denmark and Ireland have some slight similarities, these artifacts being used in conjunction with fishing nets.
The possible use of the balls as oracles has been suggested. The way in which the ball came to rest could be interpreted as a message from the gods or an answer to a question. The lack of balls found in graves may indicate that they were not considered to belong to individuals.
An alternative or supplementary use could have been as the 'right to speak' where discussions are controlled by the requirement for the speaker to hold the Carved Stone Ball or if not, then keep his or her peace and listen to the views of others. The balls are of a size that fits comfortably in one hand.
 Ball gamesSome theories are given here:
Balls were used in traditional or religious games which continue to be played today, such as the Kirkwall Ba game, which has been played on Christmas Eve and Hogmanay every year since the mid 19th century. A hand-made leather ball is thrown up in the air close to St Magnus Cathedral in Kirkwall. As many as two hundred men take part and the game can go on for several hours. The object is for the ball to reach the 'goal' of the other team, this being either at the harbour or at the top of the town. The Cornish celtic game of Hurling is still played at St.Ives and St. Columb Major and it has some similarities to the Orkney Ba' Game.
Balls of plain sandstone with the facets from shaping still clearly visible were found at Traprain Law in East Lothian. A significant number[quantify] have already been found here and are known from other southern Scottish Iron Age sites. They may date from the fourth to third centuries BC. These balls are not ornamented and do not have knobs.
Another possibility was their use in the competitive throwing of balls from one place to another, one version being Irish Road Bowling, the winner being the one who uses the fewest throws to complete the course. This sport is popular in parts of Ireland, the USA and New Zealan