Kids love rocks and they make a great addition to a Pre-K science center.
In my science center, I have a box for children to collect rocks. This box is a plastic craft box with divided sections. They typically cost $1-2. Children love to add rocks they find to the rock collection box, and this helps give children “ownership” of their classroom.
To add more variety of rocks, I also have a commercial collection (bought in a craft/hobby store) since there are many rocks of different colors and textures that cannot be found in one area. The only way to add more variety of rocks is to buy a set. Some museum gift shops sell individual rocks.