Fluorite

Fluorite is common worldwide. It is found mainly as a gangue mineral in hydrothermal veins, especially with lead and zinc ores, and also in some granites, pegmatites, greisens and marbles. Well-known gem sources include Ontario (Canada), Colombia (green), Huanzala (Peru, pink crystals), Chamonix (Switzerland, rare pink octahedra), Cornwall, Cumberland and Derbyshire (England), and Illinois, Missouri, New Hampshire, Colorado, Michigan and New Mexico (USA); it also comes from China, Germany, Mexico, Morocco, Namibia, Pakistan, Russia, South Africa and many other countries. Blue John is a banded purple, blue and white fluorite from Castleton, Derbyshire.

Fluorite is isotropic, with a very low refractive index (about 1.43–1.45) and very low dispersion, and it has perfect cleavage in four directions. Many, but not all, fluorites fluoresce under ultraviolet light, commonly blue under long-wave UV; some are also phosphorescent or thermoluminescent, so fluorescence alone is not diagnostic. A few specimens show a color change from blue in daylight to purple in incandescent light. Because of its color range, fluorite has been offered as amethyst, emerald or color-change garnet; its low refractive index helps separate it, and scratch tests should never be done on finished gems.

Treatments: heating to about 100–150 °C can lighten dark material, irradiation can darken colorless or pale stones, and decorative objects may be coated, dyed or impregnated with plastic. Some fluorite "night glowing pearls" sold in Asian markets are spheres that may be coated to enhance phosphorescence.

Fluorite is soft (Mohs 4), brittle and cleaves easily, so it is too fragile for most jewelry; rings are not recommended, while pendants and earrings in protective settings suit occasional wear. Store it apart from harder gems to avoid scratches and out of strong light, since some fluorite becomes paler with light exposure. It is slightly soluble in water: do not soak it; clean only with a warm damp cloth, mild detergent and a soft brush, and avoid ultrasonic, steam and other mechanical cleaning and heat.

Fluorspar was first described in print by Georgius Agricola in 1530 as a mineral useful as a flux in smelting; the name comes from the Latin fluere, "to flow". The name fluorite was given in 1797 by Carlo Antonio Galeani Napione. The phenomenon of fluorescence was named after fluorite by the physicist George Gabriel Stokes in 1852, and the element fluorine is also named after the mineral. In classical studies the Latin murrina (myrrhina) is identified with fluorite; Pliny the Elder described it as a stone with purple and white mottling whose carved objects the Romans prized. In England, the banded Blue John fluorite of Castleton, Derbyshire, named by 18th-century miners, was worked into vases, bowls and other ornaments.