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142
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PRECIOUS STONES.
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Fluorescence
is strongly marked in some specimens. Other effects of heat are seen,
one of the most striking being the way it decrepitates if heated
suddenly, due, it is supposed, to minute cavities within the mineral,
containing in many cases liquids. A change of colour may also occur on
heating, the blue and violet shades changing to purple. By itself it is
not readily fusible, but when mixed with some other substances it
easily fuses to a slag and on this account is largely used as a flux;
hence, possibly, the name.
On
heating, a difference of electrical potential is induced between the
cube faces and angles of the cube; this also occurs when light falls on
the crystal.
The specific gravity varies between 3'01 and 3"19.
Crystals
show a conchoidal fracture and a very perfectly developed octahedral
cleavage. On account of this cleavage, Fluor Spar may be used to
practice on to gain skill in the cleavage of Diamonds, and Tennant has
used the mineral to make models of well-known Diamonds with a view to
ascertaining the probability of the Great Mogul Diamond having been
broken up into smaller gems. On account of the ready cleavage and
natural brittleness of this mineral it is very easily damaged if
knocked or allowed to fall. The powder of Fluor Spar of any colour, if
sufficiently fine, is white, hence the mineral gives a white " streak"
when drawn over a harder substance. The hardness of Fluor Spar is the
standard No. 4 of Mohs' scale.
The
crystalline form is cubic, and crystals may have the general habit of
the cube or the octahedron, often with the edges and solid angles
highly modified by faces of other crystal forms. Crystals occur up to a
foot or more across the cube face, and in some places in enormous
quantities.
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