The dichroiscope is a handy little optical instrument, that will
readily serve to distinguish the diamond, spinel, or garnet (all
singly refracting minerals) from the ruby, beryl, or any of the doubly
refracting stones. This instrument consists of a cleavage rhombohedron
of Iceland spar, fastened in a brass tube about 2½ inches long, and ¾
of an inch in diameter. A sliding cap at one end has a perforation ⅛
of an inch square, and at the other end is a lens which will show a
distinct image of the square opening when the cap is pulled out about ¼
of an inch.
[Illustration: FIG. 1.]
The pleiochroism of many stones can be determined at a glance with the
dichroiscope.
When a stone is examined by means of the dichroiscope, it will show two
images of the same hue, or of different hues, these square images (fig.
1, A) forming a right angle and being more distinct when viewed from
one part of the stone than from another.
When the images are identical in color, the specimen may be a diamond,
garnet, spinel, or glass. Should a red or ruby spinel approach the
ruby in color, a quick and satisfactory test can be made with the
dichroiscope, as the spinel will show two images of one color, while
the ruby will show one image of aurora red and one of carmine red.
The dichroiscope is inexpensive, costing but a few dollars, and is very
useful for rapidly deciding the species of many stones. The following
is a partial list of doubly refracting stones and their twin colors.
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NAME OF STONE. │ TWIN COLORS.
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Sapphire (blue) │ Greenish straw │ Blue
Ruby (red) │ Aurora red │ Carmine red
Tourmaline (red) │ Salmon │ Rose pink
" (brownish red) │ Umber brown │ Columbine pink
" (brown) │ Orange brown │ Greenish yellow
" (green) │ Pistachio green │ Bluish green
" (blue) │ Greenish gray │ Indigo blue
Emerald (green) │ Yellowish green │ Bluish green
Topaz (sherry) │ Straw yellow │ Rose pink
Peridot (pistachio) │ Brown yellow │ Sea green
Aquamarine (sea green) │ Straw white │ Gray blue
Beryl (pale blue) │ Sea green │ Azure blue
Chrysoberyl (yellow) │ Golden brown │ Greenish yellow
Iolite (lavender) │ Pale buff │ Indigo blue
Amethyst (purple) │ Reddish purple │ Bluish purple
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