You peeked my curiosity. So I went old school....Just checked Dictionary (I know I'm showing my age)
Fluorite (also called
fluorspar) is the mineral form of
calcium fluoride, CaF2. It belongs to the
halide minerals. It crystallizes in
isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.
The
Mohs scale of mineral hardness, based on
scratch hardness comparison, defines value 4 as Fluorite.
Pure fluorite is transparent, both in visible and ultraviolet light, but impurities usually make it a colorful mineral and the stone has ornamental and
lapidary uses. Industrially, fluorite is used as a
flux for smelting, and in the production of certain glasses and enamels. The purest grades of fluorite are a source of fluoride for
hydrofluoric acid manufacture, which is the intermediate source of most fluorine-containing
fine chemicals. Optically clear transparent fluorite lenses have low
dispersion, so lenses made from it exhibit less
chromatic aberration, making them valuable in microscopes and telescopes. Fluorite optics are also usable in the far-ultraviolet and mid-infrared ranges, where conventional glasses are too absorbent for use.
Fluoride glass is a class of non-oxide optical
glasses composed of
fluorides of various metals. Due to their low viscosity, it is very difficult to completely avoid the occurrence of any crystallization while processing it through the
glass transition (or drawing the fiber from the melt).
Thus, although
heavy metal fluoride glasses (HMFG) exhibit very low optical attenuation, they are not only difficult to manufacture, but are quite fragile, and have poor resistance to moisture and other environmental attacks.
Fluoride glasses' best attribute is that they lack the absorption band associated with the hydroxyl (OH) group (3200 – 3600 cm−1) which is present in nearly all oxide-based glasses