This is recorded as a 6th century memorial stone which once had an inscription on it that read NEFROIHI 'the stone of Nia-Froich' (an Irish warrior whose name meant 'champion of the heather'. Although I failed to find any trace of this carving now.
The stone itself is leaning and about 5ft high – it would be about 7ft high if stood upright. The pool of water the stone normally stands in was frozen over so I thought I would get a closer look by standing on said ice – bad idea as my foot went through the ice and I ended up with a wet boot! The stone is visible from the road crossing the common although it is quite hard to spot as it tends to blend in with the side of the hill.
As you drive up the hill over the common, look to your right (the 'up hill' side) and you will see the stone about half way between the road and the summit. Once you spot it it is easy to walk to although when I visited it was quite boggy in places. A nice stone though and worth a visit.
The 8ft+ stone leans because a farmer supposedly tried digging under it for its fabled treasure - but a thunderstorm started up (as they so often do in these situations) and struck the stone. The farmer understandably left in a hurry.
[I'm not sure where i read this].
This [stone] was visited, in 1706, by Edward Lhuyd, who found upon it an inscription which he read, TEFROVTI; but there are no letters upon the stone at present. The upper part of the stone has been split, and the inscribed part either destroyed or taken away. This act of Vandalism is attributed by the farmers of the locality to a stone mason named Shon Morgan, who went in a fit of drunkenness to "try the quality of the stone." It is nine feet high and eighteen inches in thickness.
From 'Archaeologia Cambrensis' ser 3. v8, 1862, p134 (readable on Google Books).
On its eastern face there was an inscription, which was apparently spoilt by some miners in 1875. It was said to have read "TEFROIHI" or "TEFSOIHI". There's some argument as to how old the monument is, as it's near a Roman road, and it's been suggested that it could have been a burial marker, but perhaps at the site of a Bronze age cist anyway.
"In the letters of E. Lhwyd, preserved in the Bodleian Library, Oxford, is one dated October 10, 1693, in which this inscription is copied, and it is further stated that `close under this stone there's a small round [represented as oval] trench about 6 yards over, with a square area, &c., within it. My thoughts are that in ye area in ye midst, a man lies buried'."
CISP also sugests: "Approaching from the north the stone is on the horizon, though it's less conspicuous from other angles."