Just soak the wee devil in an acid solution (vinegar, lemon juice etc) - this will react with the *crunchy stuff* which is simply limescale from the (hard) water in your area. The acid will react with the alkaline limescale and dissolve it but you may need to experiment on times and concentrations. I live in a fairly soft water environment and only have to soak my shower head once a year but mates who live in Bath and Sussex (hard water) have to do this once a month and they leave the shower head in the acid (vinegar - any sort - is the cheapest) bath overnight.