Would it be possible to post a picture?
Shells that feel gritty (like sandpaper, almost) can be caused by a number of things that I'm aware of. Disturbance during laying, inheritance, really old or young hens, drastic change in environment or daylight hours, disease - such as Infectious Bronchitis etc.
It sounds like extra calcium deposits, to be honest. Nothing to worry about, usually the result of the egg hanging out in the shell gland too long (this would be caused by disturbance during laying) and extra calcium attaching itself to the egg. They're also called "Pimple Eggs". It seems that young hens just beginning to lay are more susceptible to these. It's also seemingly more common in the winter, when egg production begins to reduce.
Be sure that you're providing adequate nutrition and unlimited clean water.