The beautiful cubic crystals of fluorite, so many fine specimens of which grace our museums, are occasionally cut as gemstones, usually in the trapcut style and mainly for collectors, for the hardness of fluorite (fluorite is the standard 4 on Mohs's scale) is far too low to resist the wear and tear encountered in jewellery. The range of colour shown by fluorite is equalled by few other gemstones. The crystals may be colourless, yellow, brown, green, blue, violet and pink, but a true red is not known. An emerald-green fluorite probably from near Otjiwaron-go, in Namibia, has been faceted and sold under the misnomer 'South African emerald'.
Fluorite has an easy octahedral cleavage, but the highly characteristic cleavage surfaces are rarely perfect showing a stepped effect; it is this easy cleavage which makes the cutting of the crystals so difficult. The fracture, which is overshadowed by the easy cleavage, is flat conchoidal but in the compact types may be splintery. The density of fluorite is 3.18 and does not vary much from this figure in the case of crystals, but the massive types, owing to contamination with impurities, may vary between 3.00 and 3.25. The refractive index is 1.434 and the dispersion is very small (0.007 B to G). The absorption spectrum is generally vague and indecisive, but in the green variety weak bands occur at 6340, 6100, 5820 and 4450 A, and there is also a strong broad band at 4270 A which may be seen in sizeable pieces.
from "Gems, Their Sources, Descriptions and Identification," by Robert Webster and BW Anderson, 1962. Copyright © 1983.
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