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(1) Approximate immersion method.—In
this method minute fragments are immersed in a series of liquids with
known refractive indices. This could be only applied when a small
fragment can be detached from the stone. It could be done in the case
of whole stones in such liquids. A colourless stone is almost invisible
when immersed in a liquid of the same refractive index.
(2) Becke'8 method.—This
method allows the refractive index of the mineral to be matched with
that of one or other of the liquids. This method also depends upon the
total reflection of light. It is done on the polarizing microscope by
immersion method and on all small fragments. The test is fully
explained in texts of mineralogy.
(3) The Refractometer method.—This is the best, because of its accuracy and ease of operation. It can be used
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for
both unmounted and mounted stones (faceted gems). The method depends on
the principle of the total internal reflection of light. There are
several types of refracto-meters of which Tully's and Herbert Smith's
models are
34
3B
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