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The carving on the front of the stone is a combination of two ancient Christian symbols.
The circle with a four-armed cross is the Celtic or sun cross, which predates Christianity.
This is combined with the Chi-Rho symbol, where the Greek letters P and X represent the first two letters of the name of Christ, the down stroke of the P forming the upright of the Celtic cross. The transverse arm of the cross is just about visible across the centre.
The back of the stone is difficult to photograph, being very overgrown, but it appears to be an image of the crucifixion of Christ roughly incised into the stone. It appears to be later than the carving on the front, and not nearly so skilful
Local tradition claims the stone was originally a prehistoric menhir, a single standing stone, decorated with Christian symbols in the 6th century AD and placed over the grave of St Colman.
Supposedly the stone was removed from the churchyard by a local farmer who wanted to use it as a gatepost