It's those rocks that chickens hate. The advertising is SO deceptive, it really should come to a class action lawsuit.
It might not sound that serious, but actually how many chicken keepers have wasted money on that stuff that reads "Oyster Shell" only to watch it tread into the ground and still get weak eggs? It should be labeled "Limestone Pebbles of extra rough texture".
Apparently the reason behind the deception is that real Oyster shells used to be considered a mere by-product by the oyster industry. They were happy to sell them cheaply.
Then big business figured out it was a good cheaper solution to mix into concrete.
The biggest oyster providers now have contracts for all of their shell product.
So as the price rose and supply dwindled, mere pet suppliers (like Dumor) said "Let's just fill the bag with something else. They won't know the difference."
Grrrrrrrr