Fluorite is one of the most popular minerals for collectors because it occurs naturally in all shades of color. In Slovenia, fluorite crystals are found in purple and green colors. The master's thesis aimed to determine the cause of the characteristic coloration of these two variations. The obtained results and literature data imply that the color of fluorites is due to the Frenkel defect, or more precisely, various forms of it. The absorption, and thus the color, depends on the type of Frenkel defect, which is influenced by the entropy or temperature of the system. The absorption of individual defect types also depends on the elemental composition, where the lanthanides, in particular, can significantly affect the energy states of the electrons and thus the wavelength of the absorbed light (color). It has been shown that lanthanides themselves, as well as other elements, should not absorb visible light. The coloration of green Slovenian fluorites is therefore most likely caused by single and double Frenkel defects in the presence of lanthanides. The same defects are found in purple fluorite, which has a similar proportion of lanthanides to green fluorite. Unlike green fluorite, purple fluorite contains a lot of potassium, and the double Frenkel defects in the presence of potassium outnumber the double defects in the presence of lanthanides. More experimental data and more reliable theoretical analyses would be needed for a definitive confirmation of the results, however, the approaches presented in the master's thesis serve as a basis for further research.
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