The Stone Age burial chamber was discovered in September last year, by Hamish Mowatt, in the car park of the Skerries Bistro.
The discovery led to a rescue excavation, which recovered the remains of at least eight people, covering a range of ages at death, including the skull of a child about six years old. Two of the original five chambers were partially excavated and samples have been taken from them and from parts of the passageways.
The decorated stones, one of which is approximately six feet long, and was found partially buried next to the structure’s ruined fifth chamber, were found by Mr Mowatt late last year.
The stones feature incised, geometrical markings typical of Neolithic “art” found elsewhere in Orkney and throughout the country. However, because they were discovered out of context, it will require further archaeological work to confirm whether or not they actually relate to the 5,000-year-old structure.
County archaeologist Julie Gibson said: “It’s unclear, at the moment, whether the stones are Neolithic and whether they’re from that particular structure. Had they been found in situ, we could have said, without a doubt, that we had yet more examples of Neolithic artwork, but until we carry out further work, the mystery will keep us going.”