This stone is listed by the Gwynedd Archaeological Trust as a prehistoric standing stone, but Coflein describes it as the remains of two cairns (this reflects the RCAHMW records). I can also see how it might be interpreted as a natural outcrop.
I’ve plucked the name of this stone out of the sky because it doesn’t appear to have one (Plas Cregennen was a house that stood nearby until 1991 – now marked by a modern carved stone). The eight figure grid reference for Plas Cregennen Stone is SH65941450. This is based on the Gwynedd Archaeological Trust records, but having visited the site I’d say it’s much more useful than the four figure reference (at the top of the page), which is a bit misleading.