While playing with the Pockels effect of a syntethic crystal of lithium niobate using a laser I realized that the experiment could be done with a polarizing microscope in a more colorful way. Since many members of mindat are interested in crystallography perhaps they can find useful this demonstration in which you see how a uniaxic crystals turns to biaxic. I have uploaded a youtube video for those who want to observe it:

The Pockels effect consist of the change of refractive index in the direction of an applied electric field in a crystal. The one I own has ben cut so that an electric field can be applied in a direction perpendicular to the optic axis and the crystal changes from uniaxic to biaxic as shown by the changes in the interference figures.

The effect is much nicer if the interference figure is observed in a microscope instead of using a laser (which also give a nice interference figure, but without colors). A harmless way of applying the high voltage requiered to produce the effect is by using a kitchen sparker (or a high voltage power supply if available).