This might be something that most people don't have any interest in doing, but I know of at least one person who does! Not that she couldn't have figured it out on her own, of course.
Turning eggshells into calcium powder is just one way I compost all the eggshells I get from my chickens. You can put the powder in your garden, in your pets or even in you (probably should boil the eggshells first in that case though). For information on the nutrients eggshells provide and more, check here.
When I use eggs, I drop the eggshells into a bowl so I can rinse out the eggwhite that might still be clinging to the wall of the shell. I don't remove the membrane though because it contains nutrients as well. After rinsing, I dry them like this:
I know; not very high-tech, huh? Some people dry them in their oven or maybe even in their dehydrator, but by the time I use more eggs, the drying shells are bone dry and I just replace them with the new, wet eggshells. Works for me. And it's quiet.
Once dry, I put them in a freezer type bag. The plastic has to be kind of thick because the shells have sharp edges and can poke through thinner bags.
Then I use my mother's old rolling pen to flatten the eggshells. It's kinda fun. Yeah, I guess I'm a child.
They're smashed pretty well here.
I use an ancient coffee grinder (that has remnants of ground flax seeds here, I see) to grind the shells into powder. You can also use a blender or food processor, but this works fine for me since I do small batches at a time.
Here is the powder after a very short time.
And in a bowl so it can be seen better.
That was eight eggshells. I think it rule of thumb is that one eggshell equals about one teaspoon. Sounds about right.
Now I can sprinkle it around my tomato plants and any other veggie that needs a little calcium boost. Cool, huh?