Here is a link to egg shell quality.
http://edis.ifas.ufl.edu/ps020
Hens should always be offered both oyster shell calcium and a good quality grit to keep them healthy and productive.
Oyster shell to ensure good hard shells and grit to digest their food, otherwise they could possibly starve to death.
Calcium/Oyster Shells
If your hens have a good, "complete" layer feed, they may not ever need oyster shells or any supplemental calcium! However, it never hurts to offer it free choice (and it is inexpensive and lasts a long time). The reason is that some flocks that spend most of the day foraging, or are given too many treats, in which case they don't get all the calcium they need from their feed. This can cause weak or irregularly shaped eggs, slow laying and can even cause behavior problems like egg-eating, because they're so desperate for the calcium they need! (We recommend you cut down on the treats, too!)
Grit
Ever heard the saying "scarce as a hen's teeth"? That's right, chickens don't have them! Instead they eat tiny pebbles and store them in their "crop". When the food enters their crop, the pebbles grind it up to make digestion easier. For baby chicks, sand, parakeet gravel or canary gravel, available at your local pet store or grocery store pet aisle, will suffice. You can either sprinkle this in their feed or provide it in a small cup or bowl.
http://www.mypetchicken.com/backyard-chickens/chicken-care/chapter-6-preparing-for-chickens.aspx
http://www.mypetchicken.com/backyard-chickens/chicken-care/chapter-7-caring-for-chickens.aspx