Wow, had never heard of this, thank you!!! I did google it and found an article supporting this on Healthline. They also contain small amounts of other minerals, including strontium, fluoride, magnesium and selenium.

wow
that is awesome

@loplady Wow - you just brought me back! This is an old-time "remedy" used years ago and abandoned when salmonella in eggs became an issue. It was recommended to help poor child bearing/nursing women in the era where there was an saying that a woman lost a tooth for every child she bore. My grandma put crushed eggshells in every pot of boiled coffee all her life.
Here is an article about trying to introduce the practice in sub-saharan Africa, where dairy access is limited.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6221107/
and here is the article @loplady referred to:
https://www.healthline.com/nutrition/eggshells-benefits-and-risks
The takeaways from the articles - boiling/baking removes any infection risk. One eggshell provides about 380mg of calcium carbonate (39% bioavailability) plus the micronutrients. If you leave the membrane in while processing and drying, there are also tiny amounts of collage and other nutrients.

3 eggshells would be about the same as 1000mg of calcium supplement. Can someone processing a dozen or so eggs weigh the resulting powder on an accurate kitchen scale and give us a measurement (in teaspoons) of how much eggshell powder = 100mg or one gram?
What a great alternative expensive supplements, for anyone with access to enough healthy, local eggs.
Sue