An hour's walk from the carpark at the southern end of the Caban-coch reservoir. The stone is perched at 505 m right on top of the hill - keep going upwards and you can't miss it. You are almost guaranteed to see red kites flying overhead on the way.
Coflein describes it as "1.5 m high and 1.1 m by 0.8 m at the base. It is of quartz-veined rock with sandstone packing stones visible around the base where eroded from above the peat by sheep."