Why fluorite is special
Cubic crystal system + perfect octahedral cleavage + naturally clear when pure + accepts virtually any trace element to produce color. Fluorite's name gave us the word fluorescence — the British physicist George Stokes named the phenomenon after observing it most strongly in a piece of fluorite. Many specimens fluoresce blue-violet under long-wave UV.
Color and zoning
Pure fluorite is colorless. Trace amounts of yttrium, cerium, and color centers produce the famous purple-blue-green color zoning of Yaogangxian. Pure yttrium and lanthanide combinations give the deep blue of Spain's Berbes Mine and the rare pink of the Swiss Alps. Phantoms — earlier growth stages preserved as internal color zoning — record the changing chemistry of the parent fluid over millennia.
Collecting and care
Fluorite is hardness 4 — softer than steel — and has four perfect cleavages. Handle gently; a knock will produce a cleavage step. NEVER expose to direct sunlight or strong UV — purple and blue fluorite fade in months. Display behind UV-filtered glass or in a cabinet away from windows. The classic localities — Yaogangxian, Shangbao, Xianghualing, Cumbria, Berbes, Elmwood, Tennessee — each have a recognizable look.