Yes, that's a real one. It's the right kind of stone, the right kind of shape and, if I remember correctly, the right kind of lichen. Is it called Xanthoria - useful in producing orangey-brown tones in dyed wool - but that depends on which mordant is used (I think alum's best). Measure the size of the carved hole in units of 0.814" and I'll bet it's an even number. The hole won't be round and could have been used to tether an ancient pony and there's another stone beside it in the grass, it seems.