Fluorite: the Collector's Stone
Among mineral collectors, fluorite is the most popular mineral in the world next to quartz. Famed for its enormous range of colors, it is often referred to as "The Most Colorful Mineral in the World." But fluorite is not just another pretty stone; it has a number of fascinating properties and some very useful industrial applications.
Fluorite is interesting to mineral collectors because it has has several different crystal habits that always produce well formed clean crystals. The cube is by far the most recognized habit of fluorite followed by the octahedron which is believed to form at higher temperatures than the cube. Fluorite has perfect octahedral cleavage. This means that it has four identical directions of cleavage and when cleaved in the right ways can produce a perfect octahedral or diamond shape. These cleaved octahedrons are popular in rock shops all over the world. However the natural (e.g. uncleaved) octahedrons are much harder to find.
Many fluorite specimens exhibit fluorescence under ultraviolet light. In fact the very term 'fluorescence' comes from 'fluorite,' not the other way round, since fluorite was one of the first fluorescent minerals studied. The visible light emitted by fluorite is most commonly blue, but red, purple, yellow, green and white also occur. The fluorescence of fluorite is thought to be due to impurities such as yttrium or organic matter in the crystal lattice. It is not surprising then that the color of visible light emitted when a sample of fluorite is fluorescing appears to be dependent on where the original specimen was collected.
The origin of the name fluorite comes from the use of fluorite as a flux to lower the melting point in steel and aluminum processing. It was originally referred to as fluorospar by miners. Fluorite is also used as a source of fluorine for hydrofluoric acid and fluorinated water. The element fluorine also gets its name from fluorite, fluorines only common mineral.
Fluorite is used instead of glass in some high performance telescopes and camera lens elements. Fluorite has a very low dispersion so light diffraction is far less than ordinary glass, and in telescopes it allows crisp images of astronomical objects even at high power.
Fluorite is of interest as a gemstone because of its remarkable range of colors and its attractive vitreous luster. Most specimens of fluorite have a single color, but a significant percentage of fluorites have multiple colors and the colors are arranged in bands or zones that correspond to the shapes of fluorite's crystals. But it is quite a soft stone, rating only 4 on the Mohs scale, so it is not very practical for some types of jewelry, though it serves very well for pendants, earrings and brooches.
Purple or violet is the classic fluorite color, often rivaling amethyst for richness. Blue fluorite is fairly rare, and much sought after by collectors. The brilliant yellow is also quite rare. But the rarest fluorite colors are pink, black and colorless. Black fluorite is formed from exposure to radiation during the growth of the crystals. Colorless fluorite, though not particularly attractive, is prized by collectors simply for its rarity.