Anomalous birefringence is simply the result of a material's birefringence changing in a significant way as a function of the wavelenght of light. It produces anomalous interference colors in the material being examined between crossed polarizing filters.

Very thin fibers appear red between crossed polarizing filters. Thicker fibers appear blue because of the strong blue color of the mineral.

Blue wavelengths cycle more rapidly than red wavelengths. Yellow interference color begins for thinner particles and first order red appears purple because blue is increasing well before red significantly decreases. This effect changes the color sequence through the whole range of microscopic silicon carbide particles.