Cross polarization photos of Alum crystals.

Alum (potassium aluminum sulfate) is a cooking additive.  I prepared my crystal samples by dissolving Alum power in water until the solution reached saturation, then placing a few drops on a microscope slide and allowing to air dry.  Sometimes I use a coverslip to force the crystals to form in thinner layers.  Alum seems to begin crystal formation pretty quickly, within an hour you can see crystals on the surface of the solution.  You can even watch the crystals grow in real time while observing under the microscope.

Images in this post were taken using a DinoScope USB microscope.