Patterning on Westray flagstones.   Note that the patterns are very similar to the Lozenge motifs carved on the entrance stone at Newgrange . 

The top of the Altar Stone has swirl patterns and small fish debris. Match the species of fish and it would prove which formation the sandstone is from. 

Photo  English Heritage. 

Altar Stone swirl pattern ​

Rob Ixer points to the Altar Stone. 

Photo Copyright Phil Harding  

The Altar stone could have originally been part of the Ring of Brodgar circle in Orkney. Hoewever I beleive when placed at Stonehenge it was a significant keystone but also a stone for sitting on . 

The Altar Stone was gifted by the Orcadians to the Stonehenge project some 5000 years ago.  Part, if not all, of its transport was by sea. 

The Altar Stone is a micaceous fine grained  Sandstone.  Rob Ixer has looked closely at the minerology of the Altar Stone and together with Tony Clarke has now determined the stone has come from the rocks of the Orcadian Basin which includes Caithness, Orkney and Shetland.  Blocks similar to the Altar Stone are found at Birsay and in Westray, Orkney.  The slabs found intertidally in Westray Orkney some 650 miles away from Salisbury Plain are the best match.   

Here at Aikerness, Westray, slabs of stone get washed up onto the land from the sea in Atlantic gales. 

The Newgrange entrance stone has raised edges to the lozenge design, just like the fossil design seen above on the Westray stone.