Odd as this may sound, adding a third polarizer between them can cause light to be transmitted again. Some light will pass through the added filter because its polarization axis is not at right angles to the axis of the first filter. Whatever light does get through must be aligned with the intermediate filter's axis. In effect, this filter rotates the electric field. Therefore, when the rotated light reaches the final filter, some can pass through.
Most crystals also polarize light because of the highly regular arrangement of their atoms.
A crystal generally polarizes different wavelengths with different efficiencies, which causes some colors to come through more vividly than others. To capture this effect on film, position one polarizing filter above the light source and attach another to the objective lens of your microscope. Most large scientific supply houses carry such filters. Or you can experiment with making your own filters. By tuning the power supply to keep the stage temperature near the melting point of that material, you can alternately create and destroy the crystals by making small adjustments to the power.