Dawn recently had a post up about making eggshell geodes with her kiddos, and I just had to laugh as soon as I saw it because my daughter and I had been doing the very same project that very same day. Apparently Dawn and I are very in-synch when it comes to our kid-science projects! Anyway, eggshell geodes are a fun and very easy project that you can do fairly last minute, provided that you've got a few eggshells left from baking or breakfast. Here's how it works:
Eggshell Salt Geodes
Begin by taking eggshells that have been broken at the middle and trimming them slightly with a pair of sharp scissors or a serated knife to get a fairly even edge along the top.
Arrange your eggshells in an egg carton so that they have a place to sit upright when filled.
Make the salt solution by mixing Epsom salt into hot water until the water cannot hold any more salt. In other words, you will pour salt into the water (we used about a cup and a half of water, but you could easily get by with much less), mixing well with a whisk until you see that the salt is no longer able to dissolve into the water and a slurry begins to form. At this point, you can add a bit of food coloring as well.
Fill the eggshells with the salt solution. You can then set them aside to dry. This will take some number of days, depending on the humidity level in your house. As they dry, salt crystals will begin to form inside the shells, giving them an appearance similar to a geode. As an alternative, you can also put the eggshells into the refrigerator for a few hours and from what I hear, crystals will form this way as well. Nearly instant gratification! We haven't tried this method at our house yet, but I'm pretty sure that it is what Dawn did with hers, and they seemed to have turned out well.
For more about eggshell geodes, and geodes themselves, you can take a peek at Dawn's post here.