To date, the provenance of the Altar Stone is not known.  I maintain that the lithology of the sandstone in Orkney  matches more precisely than those found in the Senni Formation of South Wales.   During 2020 research was conducted identifying the sandstones in Orkney and comparing it with the Altar Stone.    The Alter Stone is a purple green micaceous Devonian sandstone of fine grains.   The closest match is found on the horizontal beds on the east side of Westray Island.  The rocks here are bedded at the same thickness as the Altar Stone. They are in block form and found intertidally and have similar surface patterning.    
The bedrock geology throughout Orkney are Devonian sandstones. To the south the rocks show vertebrate fossil fish and occasionally early plant species like horsetails. The rocks to the north show lacustrine patterning with fish bones and wind or wave patterning. Without XRF analysis, comparison of fossil patterns on the upper surface of the Altar Stone is the most accurate way of determining the origin of the stone in the Orkney Islands.

While looking at the surface patterns of the Orkney stones one soon realizes that the shapes and sizes are very similar to the Neolithic Rock Art.     Cup and Ring marks are comparable to Rain Drop fossils.  Lozenges are similar to desiccation patterns formed when the mud dries out.     All these marks are found on the sandstones found intertidally around Rousay, Orkney.