What is fluorite?
Technically, calcium fluorite isn't glass. It's actually a type of crystal produced synthetically by Canon and used in many of their top of the line L series lenses in place of low dispersion glass.
FL
Fluorite. A low dispersion mineral used as a substitute for glass in some highly corrected long focal length lenses. Canon uses most of these properties on its EF-L series long teles. Also refer to "ED".
Fluorite Objectives Describes a correction class for objectives. Fluorite lenses are semi-apochromatic, meaning their degree of correction lies between the achromatic and apochromatic.
^ "Technical Room - Fluorite / UD / Super UD glass Lenses". Canon. Retrieved on 2007-11-05.
^ "Lenses: Fluorite, aspherical and UD lenses". Canon Professional Network. Retrieved on 2008-10-04.
^ Gottermeier, Klaus.
Fluorite (a lens material used by Canon in their "L" series telephoto lenses) isn't very hard at all, and that's at least one reason why fluorite lens elements are used between regular glass ...
ED-series lenses are not sensitive to changes in temperature, so the problem of focus shift inherent in lenses employing calcium fluorite-crystal elements is avoided.
The new lens has been engineered for the ultimate in optical performance through the use of two fluorite lens elements for improved quality and reduced chromatic aberration.
These lenses are resistant to temperature changes, preventing focus shift problems in lenses that use calcium fluorite crystal elements.
Apparently this lens can carry this mark because it has a combination of both UD (ultra-low dispersion) and fluorite lens elements.
for your digital camera, consider choosing a scope that contains any or all of the following features: high definition (HD), apochromatic (APO) glass (glass that has been cleared of color and shape distortions), and extra-low dispersion and fluorite ...
 See also: Element, Lenses, Lens, Focus, Telephoto
  
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