The Alter stone lies just above the ground surface in the middle of Stonehenge.
The Altar Stone (80) is the largest of all the 'foreign stones' at Stonehenge. It is a rectangular recumbent block of sandstone, 16 ft. long by 3.5 ft. wide by 1.75 ft. deep, embedded in the earth so that its top is level with the surface, about 15 ft. within the central sarsen trilithon.
Two fallen members of this trilithon now lie across it (stones 55 and 156), and their weight has probably pressed it down to its present position. Like the adjoining bluestones, it has been carefully dressed to shape, but its exposed surface is now considerably abraded by the feet of visitors.
The naming of the Altar Stone is due, apparently, to Inigo Jones, who made the first 'plan' of Stonehenge in 1620, although the real purpose of the stone is entirely unknown.
The Altar Stone, which is a fine-grained pale green sandstone, is not from the Prescelly Mountains of Pembrokeshire, the source of the of the blue stones, but from the Cosheston Beds (a division of the Old Red Sandstone of South Wales) which crop out on the shores of Milford Haven, further south in the same county.
No other stone composed of this rock is known at Stonehenge, though occasional fragments of it, very probably detached from the Altar Stone itself, have been found in the soil of the site.
Significantly, chips of an entirely different grey-green micaceous sandstone have also been collected on the site, and have been identified with a particular outcrop of the Cosheston Beds at Mill Bay on the south shore of Milford Haven, about 2.5 miles above the ferry at Pembroke Dock.