FYI:
Fluorite is a man made (grown/cultured) crystal material that has been out for quite a few years.
Most early scopes had a one piece objective lens (Crown Glass) which alone gave a horrible amount of chromatic aberration. Early scopes used "stops" or internal rings which cut down on the color fringe but also cut down the effective amount of available light. Then came doublets or two piece objectives having a crown glass outer lens cemented to a flint glass inner lens which brought more of white light's many wavelengths closer to the same focal point. If you slipped slightly past focus in or out you would notice a color shift of red fringe to blue around the centered object.
When they developed triplet lenses by adding a fluorite lens to the other two this chromatic aberration was further reduced.
There were problems with some of the early fluorite lenses, although I cannot recall them specifically, I believe they were brittle (I could be wrong on this).
The early cement used on doublets was refined tree sap, when it delaminated it appeared as bubbles in the lens. Some types of "growth" would also contaminate these early lenses. Some tried to eliminate this problem by "air spacing" with shims of metal foil (basically tabs) around the outside of the lenses. Drawback to this was light and contrast lost to internal reflections from every air to glass surface. Multicoatings made great progress by cutting down the amount of reflected light.
Optical designs incorporating mirrors greatly reduce the amount of chromatic aberration as well as quite a few other issues of conventional refractor scopes, but at the cost of depth of field.
Hope this helps, and yeah, my spellings only aprox.
Take care,
warren
Edited to add: The reason you do not see gigantic telescopes with these composite lens elements is lens sag, the glass actually flexes adding distortion if you go past 40 inches or so in diameter.
