To those that raise chickens. Have you ever gotten an egg that looks like the one on the left? It is white chalky and somewhat scaly. I rinsed it off in cold water and it turned brown but when it dried it looks like the picture.
I've had hens occasionally lay them. The pigment wasn't laid down correctly and it nothing to worry about unless it is more brittle. Always keep a separate bowl of oyster shells available.
They will eat the oyster shells as needed, so mixing them with other feed isn't necessary. It's easier to keep them separated and refill the bowl as necessary.
I keep my oyster shells in one of those vertical pipe feeders, i like it because it holds a lot and is out of the way in my run. Most of my hens feed throughout the day at it randomly but I have one hen that takes a few bites every night on her way to bed.. it cracks me up every time, that girl never skips a beat.