It is possible for crystals to also be present in the urine and be a sign of a medical problem. Please have the urine checked for crystals by your veterinarian. The sample should be fresh within 1/2 hr or be collected by your veterinarian for optimum results.
Urine contains waste products which may normally include dissolved minerals. As moisture evaporates the solution becomes more concentrated and these minerals come out of solution as crystals. Crystalline material refracts light and will appear to glitter when viewed with an appropriate spectrum of light and angle.