|Transparency||Transparent to translucent|
Fluorite (FLU-oh-rite) is included among gem minerals not because it is frequently faceted, since its low hardness and brilliancy and excellent cleavage make it impractical for this purpose. but because the variety called "blue john" has been used widely for carved ornamental pieces. The name of the species is derived from the Latin word "fluere", meaning "flow". At first, the name was applied to all minerals that fused easily and that could be used as fluxes; later, however, after the discovery of hydrofluoric acid, it was used only for those that contained fluorine, and eventually for the principal one, fluorite. The mineral is transparent to translucent and occurs in a wide variety of attractive colors, including red, green, blue, brown, yellow, orange and violet. It is usually, but not always, light in tone. Perhaps the most interesting characteristic of most varieties is the very strong fluorescence under ultraviolet light.
Fluorite, which is a fluoride of calcium (CaF2), occurs throughout the world. It is rather commonly found in what might be regarded as gem quality. The finest specimens come from Derbyshire, Cumberland, Cornwall, Devonshire and Durham in England. Other noted localities are the St. Gothard and Chamonix regions of Switzerland, Saxony, Norway, South-Nest Africa, Australia, New York, Illinois and Kentucky. It is also found in the western United States. Most commonly, it is a mineral that occurs in association with such metallic ores as lead, silver and zinc. It is also found with quartz and calcite. It is usually formed as a result of the action of magmatic solutions operating at some distance from the source.
Fluorite occurs in the cubic system; therefore, it is isotropic, or singly refractive. The crystals are usually cube shaped; less frequently they are octahedral form as that of diamond. The luster is sub vitreous, the hardness is 4, the fracture is conchoidal and the toughness is very poor, except in the massive material used for carving. The specific gravity is 3.18 and the refractive index is 1.434, the lowest, excepting that of fire opal, among the minerals used for gem purposes. Its dispersion, .007, is very low also. It fuses readily before the blowpipe or the jeweler's torch and is decomposed by sulphuric acid. No phenomenon is shown. A highly characteristic feature of fluorite is triangular shaped negative crystals containing a watery liquid and a movable bubble. These inclusions may be found singly or in great quantities; they are not confined to stones from any one locality, but are found in material from all known sources.
Identification is a comparatively simple matter, because of the low refractive index. Glass is easily separated because of fluorite's usually apparent cleavage and its natural inclusions. The strong ultraviolet fluorescence of many specimens is also an identifying characteristic.
Transparent material may be faceted, but it often proves difficult. Because of its sensitivity to heat, low hardness and perfect cleavage, great care must be exercised in all phases of the fashioning operation. Stones must be oriented so that none of the facets are placed parallel to a cleavage plane. A satisfactory polish can be obtained by using Linde A powder on a tin lap; an alternate method is a wooden or wax lap with tin oxide. Suggested angles are 42° for the crown and 43° for the pavilion.
Perhaps the most common use of fluorite in jewelry is an imitation of cabochon-cut or carved emerald. Sooner or later, the excellent cleavage gives trouble and the identity of the stone is questioned. Another use is in imitation of amethyst in cabochon form; tourists frequently buy them mounted in silver jewelry in Mexico, assuming them to be amethyst.