The kids looked through a shoebox full of different rocks I collected from the beach and from other areas of Lanai. I then added in a collection of crystals and some other Earth Materials (driftwood, kukui nuts, coral, and others).
They then sorted everything into categories that made sense to them. They had to name their category and explain/justify it, as well.
Here is a collection of pumice - a volcanic, igneous rock.
We even found books at the library on rocks and crystals!