Fluorite is a naturally occurring crystal with three special properties that make it eminently suitable for use in lenses − it transmits infrared and ultraviolet light well, has a very low refractive index, and has low dispersion.
What does this mean for your images? When light passes through a lens, it refracts – that is, it bends. It also breaks up into its constituent colours, just like light passing through a prism. The lower the refractive index of the lens material, the less the light bends and the sharper the image. Similarly, the lower the dispersion ratio, the less the light is broken up, which makes it easier to correct chromatic aberration.
Chromatic aberration is inherent in glass elements. The problem occurs because the lens is not able to bring all the different colours (wavelengths of light) to the same focus point and, in the worst cases, coloured fringing is seen along some edges. A fluorite lens element has a lower refractive index, which reduces the effect.