Hematite has a complex refractive index of 2.937 + i(0.24268) for epsilon. The omega refractive index is about 3.2 + i(0.1) at the same wavelength. When using transmitted light hematite appears nearly opaque. With its high birefringence, approximately 0.28, even small particles appear red between crossed polarizing filters. It appears red because it transmits red wavelengths much more efficiently than the shorter wavelengths (blue, green, yellow). With Brightfield illumination the background is too bright to see the small amount of red light transmitted. With crossed polarizers the background is dark and the light transmitted as a result of the high birefringence of hematite is red.

Polished sections of tourmaline were the first linear polarizing filters.