If you don’t have a cap on top of your chimney, you need one. This keeps moisture from creeping down the chimney and then freezing and popping off bricks, rock, whatever….then the entire chimney needs to be sealed with an oil based (preferred over water-based) sealer. We had this issue for years before realizing we needed the cap on top…no more problems, but thanks for the reminder for us to re-seal our rocks again. We used someone local but sorry, don’t remember who!