The Glan-Thompson prism is a calcite crystal that has been cut at specific angle and then rejoined so that the ordinary and extraordinary rays are at 90 degrees polarization to one another.  Our crystal has been cut at a 30-degree angle which is not optimum but usable.  We also had to have two of the surfaces polished after purchase which will give some spot distortion, but the crystal is completely usable. Align the Metrologic unpolarized laser into one end of the crystal and turn the angle of the crystal slightly until two good bright spots appear on the screen.  Insert the polarizer and show that by turning the polarizer about 90 degrees, you can shut off one of the beams or the other.