The best replacement for oyster shell is calcium carbonate.

The best alternatives to oyster shell are rated as following:
★★★★★  calcium carbonate
★★★★  calcium citrate
★★★  grit
★  chicken grit

Your alternative for oyster shell is not listed? Suggest!
Speaking about oyster shell, one probably shouldn't forget about calcium.

Other search terms related to oyster shell and their ratings are:

★★★★★ calcium
You have in mind a companion for oyster shell and it is not yet here? Propose!

Google Trends

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oyster shell
calcium carbonate
calcium citrate