Marcia,

Thank you for your question. 

There is a lot of information about using eggshells to improve the calcium in our soils. Colorado soils contain adequate calcium for all types of gardening already. If you suspect you may have a calcium deficiency in your soil, you should have a soil test done. The results will give you information about the health of your soil. In fact, it is good to have a soil test done periodically. Here is a link to information regarding soil tests:
https://planttalk.colostate.edu/topics/soils-amendments-composting/1606-soil-tests/

If you have experienced Blossom End Rot in your tomatoes, some sources say to add eggshells to increase the calcium. It is not that your soil lacks calcium that causes blossom end rot, it is the plant's inability to pull up water and nutrients from the soil. Adding eggshells will not prevent or solve this problem. 

There is nothing wrong with adding eggshells to your garden soil or you can just add them to your compost pile if you have one. Because eggshells break down very slowly, it is better to clean them thoroughly, dry them, and then grind them up in a blender or coffee grinder so that the pieces are very small or are powdery. This way they will break down more quickly. Adding eggshells will not change the pH of your soil. 

Here is a CSU Fact Sheet that will help you to choose an organic  soil amendment:
https://extension.colostate.edu/topic-areas/yard-garden/choosing-a-soil-amendment/?_gl=1

I also found this from University of Minnesota:
https://extension.umn.edu/manage-soil-nutrients/coffee-grounds-eggshells-epsom-salts
Even though it is written for Minnesota soils, the basic information is relevant for CO soils as well. I like this article because it addresses the most common internet garden "myths" about adding coffee grounds, epsom salts, and eggshells to your soil. This information is research based and credible. 

I hope you find this information helpful. If you have any questions you may reply to this email.
Debbie M.