Chromatic aberration happens because of dispersion --- the fact that the refractive index changes with wavelength. By combining materials with different propert

Chromatic aberration happens because of dispersion --- the fact that the refractive index changes with wavelength. By combining materials with different properties (e.g. low dispersion, or high refractive index), you can cancel out the aberration in lens designs such as achromatic doublets, apochromatic triplets, etc. In general, materials that are high in refractive index have high dispersion (e.g. crown glass) and materials that have low dispersion also have low refractive index. But ideally we want a material that has high refractive index but low dispersion so that it can both bend light without introducing a lot of chromatic aberration. Fluorite has extraordinarily low dispersion while having a refractive index that's only slightly lower than glass, making it a good material to be used in conjunction with other materials.