Looks nice but this is why I bought it:
"...if unpolarized light enters the material at a nonzero acute angle to the optical axis, the component with polarization perpendicular to this axis will be refracted as per the standard law of refraction, while the complementary polarization component will refract at a nonstandard angle determined by the angle of entry and the difference between the indices of refraction, known as the birefringence magnitude. The light will therefore split into two linearly polarized beams, correspondingly known as ordinary and extraordinary."In other words, light of a one polarity will pass the crystal at one index of refraction while light polarized at the complementary angle (∆ 90° or 270°) will be refracted at a different index. The result is two separate images. The relative position of the secondary image to the primary image changes as the crystal is rotated.
Here is a sequence of images taken with a circular polarizer which only transmits light of a specific polarity. The word "Calcite" changes position as the polarity is changed from that of the "ordinary" index of refraction to the "extraordinary" index of refraction (or vice versa). The angle of polarization is increased ~22.5° in each image:
I've always wanted a nice piece of calcite to play with. I bought it from mineralmatrix on eBay and I can highly recommend them.