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.    The Alter Stone is a purple green micaceous Devonian sandstone of fine grains.   The closest match is found on the intertidal horizontal beds on Papa Westray and the east side of Westray Island. These are the most northerly islands of Orkney, a place where the Megalihic builders had been living 900 years before Stonehenge was even conceived.  The rocks here are bedded at the same thickness as the Altar Stone. They are in block form and found inter tidally and have similar surface patterning.    
The bedrock geology throughout Orkney is 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.
While looking at the surface patterns of  the Orkney stones one soon realizes that the shapes and sizes are very similar to the Neolithic Irish / Celtic 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 Westray, Orkney.