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Emily,  ref. quick crystals:

Naphthalene crystals form very quickly on cooling - put some on a glass
slide and heat it - then let it cool - they form in a minute or so. If you
watch it through a polarizing microscope, you get a great color show.

Another possibility - make fudge!  In fact, you can make quite a show of a
cooling sugar solution - one that boils at 240 Fahrenheit.

If you give it a short stir (to form a few nuclei) and then let it cool
slowly, large crystals of sugar form.  

If you stir it a lot (to make lots of nuclei) and then let it cool fairly
quickly, a fine-grained texture results - this is what you want for fudge.

If you pour the boiling mixture on a marble slab to supercool it, you get a
very heavy syrup.  If you work this with a spatula (it's very hard work!)
you get fondant consistency  (that you reheat slightly before pouring into
a fondant mold.)  If you keep on working the findant mixture, you get icing
sugar - or somethng like it!

I am  geologist, so this really strikes a chord with me.  The slow cooling
gives coarse crystals like a pegmatite.  The fudge process gives something
like a lava,  In between, you can get coarser grains like a granite.  The
supercooled mixture is like a volcanic glass, many of which crystallize
later, slowly. 

Hope this helps.

Good luck!  

Peter Anderson

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