Crystal Models

Minerals are made of a crystalline structure.  Many students don't really know what that means.  And a verbal explanation won't suffice for many middle school students.

Have the students make models of the basic crystal shapes using these paper models (or if you don't have the class time to devote to this, you could make a set to have on hand when it comes time to explain crystalline structure*):

Cubic Crystal
Tetragonal Crystal
Hexagonal Crystal
Orthorhombic Crystal
Rhombohedral Crystal
Monoclinic Crystal
Triclinic Crystal

*If you're making a set for classroom use, to have on hand year after year, you might want to go to the extra effort of making your models with cardstock.  They take a little more effort to fold, but they'll hold up better.  Also, make sure you label your models with the crystal shapes - you may be able to remember them for a couple of days, but when you pull them out next year... :)