I've driven past this War Memorial on numerous occassions, and apart from thinking it was made from a remarkably large lump of quartz, never thought anything of it.
More recently I started to wonder about the origins of the this boulder. It reminded me of some of the standings stones in the area (the Glandwr Stone and one of the Pond Nant y Cagal stones). I made some enquiries and a good friend of mine who lives nearby (and has a passing interest in archaeology) assured me that this was probably just a boulder transported from one of the nearby mines.
It was only by coincidence that while I was thumbing through some records in the NMRW looking for something entirely unconnected, I found an the extract from Sainsbury's thesis (below). This indicates that the stone does have a mining connection, but it wasn't dug out of the ground by modern miners, it was already standing out in the open. I can't help feeling a bit smug.
According to a thesis by AR Sainsbury (published in 1939) the quartz stone that acts as a war memorial in the village Penrhyn-coch was originally a standing stone (standing at SN66138586).
A large menhir of white quartz formerly stood on the crest of the ridge north of the road and 400 yards north-east of the magazine of Court Grange mine. It has since been placed on a plinth and adorned with a cross in the village of Penrhyncoch and now forms the village War memorial. The stone is approximately 5' in height and 15' in girth and is rectangular in section.
Apparently the stone was marked on the 1853 OS map, presumably in it's original location. The stone is sometimes referred to as the Banc Troed Rhiw Seiri Stone, in reference to its original location.