fluorite

Definition: Calcium flouride crystal in form suitable for optical use. * Characterised by low refractive index, very low dispersion. * Also transparent to infrared and ultraviolet.
Fragile flourite, untouchable performance
Flourite is highly favoured for its ability to help produce apochromatic correction in longer focal length lenses. However it is extremely difficult to fabricate: difficult to grow and very fragile to polish. It also must be protected from the environment being soft and water absorbent. Canon have taken the trouble, however, and its lenses with flourite elements are amongst the very finest photographic optics anywhere.

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