The monument comprises a particularly fine example of a Pictish symbol-bearing cross-slab. Dating to the late 8th century, it is no longer in its original position but it is earthfast and still associated with its findspot, the now redundant 17th-century parish church of Nigg. Historic Scotland's predecessor body was responsible for relocating the cross-slab in 1978. The monument was first scheduled in 1925, rescheduled in 1993. However, there is a discrepancy between the scheduled map and the description and the present scheduling rectifies this.
The cross-slab is an upright, rectangular sandstone slab with a pedimented top, 2.2m high by about 1m wide, sculptured in both high and low relief on front, back and right side. The front bears a cross with complex relief interlace work, set in a field bearing bosses which are also decorated with, and surrounded by, interlace. Above the cross, at the apex of the stone, there is a scene depicting SS Anthony and Paul and the raven. The reverse of the stone was badly damaged in antiquity but there is evidence of an eagle above a Pictish beast with, below, a hunting scene including a hound, a deer, a mounted figure and a standing figure. Within the hunting scene is a representation of the biblical King David killing the lion.
The cross-slab was broken sometime in the past (the stone is believed to have been intact until 1725 when it is reputed to have been blown over in a storm), and a section which measures between 16cm and 26cm is now missing. The missing section is filled with a concrete. However, despite the loss of the portion of the cross, it is relatively intact. In 1998, a piece of the missing section of the cross-slab was found in the bed of a small burn a few metres below the E gable of the church. This has not been physically reunited with the cross-slab.
The cross-slab is one of a series of outstanding carved stones of 8th/9th century date found in Easter Ross. Stylistically, it is particularly closely related to the stones at Shandwick (in situ) and Hilton of Cadboll (now in the Museum of Scotland/Hilton), and the smaller fragments from Portmahomack. As a group they suggest the existence of a local school of carvers connected to ecclesiastical centres of some description, with considerable patronage from the local aristocracy. On stylistic grounds the Nigg cross-slab is believed to stand between the Hilton of Cadboll and Shandwick cross-slabs, and is perhaps the most accomplished of all.
The carving on the cross-slabs demonstrate the many wide artistic, iconographic and cultural connections that this area of Scotland had with the rest of the British Isles and beyond to Continental Europe. It has been suggested that the influence of the Easter Ross school extended as far as Iona; the prominent bosses, covered in interlace work and encircled by snakes, of St John's and St Oran's crosses appear to have been influenced by the Nigg cross-slab. The overall design of Nigg, front and back, shows an orderly and symmetrical balancing of panels of decoration, and this use of panels becomes a distinctive feature of later cross-slabs.
The icongraphic programme of the Nigg Stone has distinct similarities to that of the St Andrews Sarcophagus, and has huge potential to inform us about the nature of Christianity and its relations to the secular world of the Pictish kingdom. Strong stylistic parallels suggest that the St Andrews Sarcophagus and Nigg Cross-slab were carved by sculptors trained in the same school, if not the same sculptor.
The area to be scheduled is a rectangle measuring 1.3m by 0.4m, to include the cross-slab, its modern mounting and associated fittings, both above and below ground. The detached portion discovered in 1998 cannot be scheduled because it is portable.