# What does the refractive index for e.g. alpha mean?

When I look for some materials like https://en.wikipedia.org/wiki/Barium_sulfate and want to extract the refractive index then there is written: (nD)=1.636 (alpha). And sometimes also for beta and gamma.

What does this mean? The refractive index is mostly dependent on the wavelength so why is alpha, beta and gamma the only value given?