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. Balls of plain sandstone with the facets from shaping still clearly visible were found at Traprain Law in East Lothian. A significant number 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. In 2013, archaeologists discovered a carved stone ball at Ness of Brodgar, a rare find of such an object in situ in "a modern archaeological context". Early, undecorated, polished stone balls have also been found 'in-situ' inside a 5,500 year-old tomb at Tresness on Orkney by Professor Vicki Cummings. Physical characteristics Materials Many balls 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. Including Old Red Sandstone, 43 are sandstone, 26 greenstone, 12 quartzite, and 9 were serpentinite; these had been carved. Some were made of gabbro – 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 have not as yet been fully inspected or tested to ascertain their composition. Experimental archaeology Full replicas have been made using authentic manufacturing techniques (pecking and grinding) by A.T. Young. It was shown that they could be made using prehistoric technology with no recourse to the use of metal tools. Size, shape, and knobs Of the 387 carved stone balls known in 1976 (now about 425), 375 are about 70 mm in diameter, but twelve are known with diameters of 90 to 114 mm. Only 7 are oval. They are therefore about the size of tennis balls or oranges. Nearly half have 6 knobs. {| style="text-align:right;vertical-align:bottom;" |+ |-style="text-align:right;" ! Carvedportrusions ! Numberof finds |- | 3 knobs || 3 |- | 4 knobs || 43 |- |