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PRECIOUS STONES 125
" Opalescence" is a milky or pearly reflection from the interior of a stone.
" Iridescence" describes the prismatic colors seen within a crystal.
" Dichroism," from the Greek dis, twice, and chroa, color,
implies different colors in two directions. It is a property of all
doubly refractive stones, of which the two images shown by the
dichroiscope appear in different colors. The " twin-colors" are these
two colors. " Pleochroism," from the Greek pleos, full, and chroa, color, applies to minerals in which a different shade of color is seen in more than two directions.
" The index of refraction" is the angle to which the ray of light is bent upon entering the stone. In water it is 1.335 ; in
quartz, 1.548; in zircon, 1.961; in diamond, 2.439. The index for both
rays in doubly refractive stones is given in all cases where a record
of measurement could be found.
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