Now I can see the side, I agree with Pete and Gary, that it resembles chert. How those strange features on the top/base came about, I wouldn't like to guess without close inspection. I suppose it is still possible that some soft sediment movement brought about the features you can see, but I am also in agreement that a good use would be to enhance your herb garden! As for the host formation...Chalk may be a possibility, particularly if it is from the Northern UK Province, but I would favour the Upper Greensand of the general Dorset coast area, where the banding would be commoner. In truth, it could have been picked up almost anywhere.