That stone looks awful clean to have been buried all those years. My first thought was maybe it was a caricature of a Pict, but since it doesn’t appear antiquated, I’m not too sure about it.
The problem is it was all mossy when they found it and, unthinkingly, they cleaned it up before bringing it in.
All washed and wire-brushed. I don’t know how any lab could get around that.
Microscopic nooks and crannies?