The boulder itself is substantial, roughly four metres across on its longest axis and rising to about 75 centimetres at the north-west end, with a flattish, table-topped surface that bears the decoration. What makes the surface visually striking, if you know what to look for, is its bleached appearance: the overlying sod, about 40 centimetres deep, was stripped away in relatively recent times, leaving the stone exposed and pale against the surrounding heath.

The carved motifs belong to the prehistoric tradition known as rock art, in which cup-marks and rings, those small circular depressions and the concentric grooves encircling them, were picked or pecked into stone surfaces, most likely during the Neolithic or Bronze Age. At Kealduff, the south-west-facing aspect of the boulder holds the densest grouping. There is a cup-and-two-ring motif on the eastern side of the decorated surface, and a smaller cup-and-ring adjacent to the south. A picked linear line, 50 centimetres long, connects that smaller motif to a cup-and-three-ring, the largest of the group at 23 centimetres in diameter, whose central cup-mark has been very deeply eroded over the millennia. From this motif, another picked linear groove extends westward, forking as it goes: one branch runs to a faint cup-and-ring to the north-west, the other continues in a curvilinear path along the western edge of the boulder before fading out entirely. A separate curvilinear groove is also present on the south-east-facing aspect of the stone. The connecting lines between motifs are relatively unusual and lend the surface a quality of deliberate composition, as though the maker intended the elements to be read in relation to one another rather than as isolated marks. A second recorded rock art site lies less than a metre to the south-west, making this a small but concentrated cluster within the broader Behy river valley landscape, with the Seefin and Droum Mountains visible to the north-east.