If you, like me, love your happy-hen eggs, and yet you are ditching the shell, or just using them around the garden then you might want to stop, because home-produced eggshell powder is an amazing natural calcium supplement and can be made for free.
And what’s more, a study in isolated cells found that calcium absorption was up to 64% greater from eggshell powder compared to pure calcium carbonate.  (It’s helped by the other stuff in the shell, including the delicate lining). While in other animal and human tests, eggshell calcium shows increased bone density, less arthritic pain, and even stimulates cartilage growth. 
So yes, if you need or already buy calcium supplements produced by big industry, then you are paying for something way less effective than something you can make at home, without any complicated jiggery-pokery, and without inheriting a plastic pot destined to choke earth. Win-win and win again.
And while calcium may be the most abundant mineral in our body, mostly in our bones and teeth, it is also needed for other essential functions, like muscle contraction and cell formation. The bad news is, if we are becoming calcium depleted, our bodies start to leach it from those same bones and teeth I was just talking about. I am having nightmares of crumbling teeth and dental extraction as I type… quick, lets think about those eggshells again!
All you need is a bit of boiling, followed by a bit of oven drying and a whizz in a blender and ta-dah! And with the beauty of the internet, there are now plenty of tutorials on uTube, which I highly recommend you check out. I found this one, but there are plenty of others too.
Let me know in the comments if you try it, and any ways that you come up with for including it in your diet. Stay healthy and green, Dx
 Lucas R. Brun, Maela Lupo, Damián A. Delorenzi, Verónica E. Di Loreto & Alfredo Rigalli (2013) Chicken eggshell as suitable calcium source at home, International Journal of Food Sciences and Nutrition, 64:6, 740-743.