the
mineral to prevent cracks starting and portions breaking off. The
cleavage also produces flaws in the stones. Crystals of fluorite
generally have the form of simple cubes. These cubes are sometimes
modified by other forms, and twinned cubes are not uncommon. One of the
most unique and pleasing properties of fluorite is that known as
fluorescence. When pieces of the mineral are heated gently their
interiors light up with a bright glow, usually colored, and this color
is quite independent of that of the mineral—a blue fluorite, for
instance, often exhibiting an emerald-green fluorescence, a green
stone, a purple, and so on. Sometimes the heat of the hand, or the
striking of two pieces together, is sufficient to excite this glow, and
an increase of heat may cause it to change color. If heated too highly
or too long the mineral loses both this property and its inherent
color. The cause both of the color and the fluorescence is undoubtedly
hydrocarbons which exist in the mineral.
Fluorite
is a simple fluoride of calcium, having the percentage composition
fluorine 48.9, and calcium 51.1. It fuses rather easily before the
blowpipe to a white enamel, which gives an alkaline reaction. Its
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