You might want to try some zinc oxide crystals. They form amazing three dimensional tetrahedral structures. I saw them with a SEM a lifetime ago.Aside from the whole 'very toxic' thing this sounds fun.
You can make it by igniting a thin strip of zinc metal (rescued from the shell of a dead cheap carbon-zinc battery). It burns like magnesium, but with less enthusiasm.
The thick white smoke is very toxic to the lungs, so don't breathe it. It should stick to a glass microscope slide without much encouragement.
What do you think we should image?