I throw in egg shells all the time. I hope it's Okay.
This is from the Insinkerator web site.
"Do grind food waste with a strong flow of cold water.
Do grind hard food waste like meat bones, fruit pits, etc. The hard particles create a scouring action inside the disposer�s grind chamber that is good for it.
Do grind citrus and melon rinds.
Do dispose of coffee grounds in your disposer. Coffee grounds will not harm the action of the septic tank.
Do dispose of small amounts of fats and greases in your disposer. Use a strong flow of water. If you have a large amount of grease, allow it to solidify and dispose of it in your trash.
Do flush the disposer to clean it. Allow the disposer and cold water to run after it has finished grinding or after you drain dish water.
Do tell your friends about In-Sink-Erator�s great disposers.
Don�t use hot water to grind food waste.
Don�t turn off the disposer or water until the grinding is complete.
Don�t pre-load (put the food waste in the disposer before you turn it on) - unless you have a batch feed disposer.
Don�t grind extremely fibrous food waste like corn husks, artichoke leaves, and raw chicken skins. They don�t grind very well and can cause drain blockage.
Don�t grind large bones, clam shells or oyster shells.
Don�t use chemical drain cleaners in the disposer. If you think you have a drainage problem, follow the instructions for water draining slowly.
Don�t grind glass, china, plastics, paper, plastic wrap or bags.
Don�t grind metals, such as bottle caps, tin cans, aluminum foil, or silverware. Undoubtedly, you will accidentally do this and then you will need to fix the jam.
Don�t become alarmed if the face of your grinding disc becomes brown and discolored. This is a surface discoloration only and does not affect disposer performance. "