When I have taught geology at a Children's University, (Saturday school for the stuff they won't let us teach anymore), I have raided the sweetie shops and used toffee etc for teaching rock textures. Cinder toffee comes out as lava, toffee as obsidian, and fudge is something fine-grained like dolerite. Then we eat the specimens. Can't do that with real rock: that would really get the teeth.

Penny

PS. I make my own granite with coffee crystals. Anyone any idea how to do sedimentary rocks?