Homemade Dinosaur Eggs/ Geology eggs

Geology eggs, paleontology eggs, dinosaur eggs, gem eggs, dig your own fossil eggs... You know the type. You get the kits where, with a small tool, you can dig a treasure of some sort out of the plaster mix.

I thought they'd make a terrific party favour for Miss 4's birthday party, however the bought ones cost too much. So here's how we made our own.