Have no evidence - only intuition - that these might be linked:

http://www.themodernantiquarian.com/site/8971/cefn_gwernffrwd.html

- a similarly out of place quartz boulder in a similarly elevated Welsh position (currently obscured by forestry).

Btw: please visit these sites if you can - they are very remote, beautiful and sadly neglected - the circle may be gone in a few years.

t.