There are two lens terms that have similar sounding (and potentially confusing) names: fluorine and fluorite. Apparently quite a few people have trouble distinguishing between the two, so here’s a quick look at what they each are.
When lens companies talk about fluorine, they’re talking about a special coating based on the element fluorine that’s applied to the surface of your lens glass that helps to repel things like dust, water, grease, and dirt.
Here’s a short demonstration video by Nikon that was shared earlier this year:
Any mention of fluorite refers to the mineral used to create the lens elements inside the lens. It’s a monocrystal that has low chromatic aberration while being much lighter than traditional optical glass — in other words, higher optical quality with less weight.
This video on Nikon lens technologies briefly touches on fluorite lenses at around 1:32:
One easy way to remember these meanings is to understand the suffix “-ite”, as in fluorite. It’s from the adjectival form of the Greek word “lithos”, and it means rock or stone. Think “meteorite.”
(via Nikon Rumors)