Hard shells are from a good diverse diet, it just goes to show she is not educated on chickens.... so pull her aside and explain to her why the egg color changes and why they are so hard to crack... and you might have yourself a loyal customer.
Plus the younger the hens.... the harder the shells. The more eggs they lay the more brittle they become.
Plus the younger the hens.... the harder the shells. The more eggs they lay the more brittle they become.