Rock Cycle [More important than you think! When picking rocks for hardscapes you want to use Metamorphic rock in most cases, especially with water (so it doesn't leach anything into the water) but it is also important to know what type of rockscape your critters may have evolved with]
This activity is a perfect follow up for a middle school class who has just been introduced to the rock cycle. You will need starbursts (or other fruit chews of multiple colors) and plastic sandwich bags. Students begin with 'sediments' which they turn into a sedimentary rock, apply heat and pressure to turn it into a metamorphic rock, and then you (the teacher) will need to take them home to turn them into igneous rocks.
Schist is metamorphic rock with well developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces. It is a rock of intermediate metamorphic grade between phyllite and gneiss. The specimen shown above is a "chlorite schist" because it contains a significant amount of chlorite. It is about two inches (five centimeters) across.