Birefringence (double refraction) is a fundamental optical property of anisotropic crystalline substances, where crystals belonging to certain crystal systems exhibit it while isotropic crystals (such as those in the cubic system) do not; therefore, no citation is needed.
The claim states that certain salt crystals exhibit birefringence while others do not. This is a well-established textbook fact of mineralogy and crystallography: crystals belonging to the cubic crystal system are optically isotropic and do not exhibit birefringence, whereas crystals in the hexagonal, tetragonal, trigonal, or lower symmetry systems are optically anisotropic and exhibit birefringence. This qualifies as common knowledge regarding crystal optics.