Students must write down 3-5 (depending on the students) observations about each mineral: state of matter, color, shape, shiny-ness, etc.
For homework, I have students use their observations to draft their own definition of what a mineral is. They're looking for things that all of the minerals they observed have in common. At the very least, they should get "solid" as part of their definition.
I then have them look up the textbook definition and we proceed from there.
