Hmm..
Could be disease. Respiratory infections such as Infectious Bronchitis will not only attack the respiratory tract, but the reproductive organs as well. Could be something less serious, such as a disturbance during the laying process, inheritance. Drastic change in environment and daylight hours could cause the hens to lay wonky eggs. Be sure you're providing adequate water and nutrition as well. 
 
[FONT=Verdana, Arial, Helvetica, sans-serif]I suppose it could be calcium deposits, but these do not usually feel as you're describing.[/FONT]