Let me give you some of my thoughts. Oyster shells are what all of us use. I included. I have never seen other shells crushed up and sold, such as clam shells. There is a supply of them, but apparently no demand for chicken calcium supplement from such. Zebra shells would fall into same category. If you break a clam shell, you will notice a very sharp edge. Years ago peeps used to remove hair with as well as to scrape fur/leather for processing. Cracked pieces would be almost like cracked glass. It may not be the best for digestive track of a chicken.
Ouster shells are not that sharp and are quite soluble in the chickens digestive track. The gizzard also use, to grind up the food chicken eats. The oyster shells work as a grit, but do not last as long as stone grit. A bag of oyster shell goes a long way. It is not very expensive, but that is a relative term tied to ones budget.
Years ago when oyster shells were not readily available commercially, chickens got their calcium intake from free range feeding. Calcium is in much green vegetation as well as limestone granules/pebbles that chickens find scratching.
If you are feeding commercial feed, you may, or may not need to supplement calcium. If all of your chickens are laying, and you are using a Layer feed, then most likely you are OK. Layer feed is at 4% calcium. If some of your chickens are laying, and some not, including roosters, it is better to feed a lower percentage. I use Alflock which is at 2% calcium. I do offer Oyster shells free choice, as well as all the egg shells that we save and crush back for the chickens. Extra calcium free choice is always a good idea regardless which feed you use.
WISHING YOU BEST,,,,,, and
. Granite Grit next to second water source under raised coop.
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