the
stone onto the card, observing whether the spots of light are single or
double.

The table below gives the necessary information as to which stones show
double and which single refraction.

TABLE GIVING CHARACTER OF REFRACTION IN THE PRINCIPAL GEMS

_Refraction Single:_
Diamond
Garnet (all types)
Spinel
Opal
Glass
_Difference between
highest and lowest
refractive indices_
_Refraction Double:_
Sphene .084
Zircon .053
Benitoite .047
Peridot or chrysolite .038
Epidote .031
Tourmaline .020
Kunzite .015
Ruby and sapphire .009
Topaz (precious) .009
Amethyst and quartz topaz .009
Emerald and aquamarine .007
Chrysoberyl .007

The student should now put into practice the methods suggested in this
lesson. Look first for the visible doubling of the lines of the back
facets in peridot (or chrysolite); then in zircon; then in some of the
less strongly doubly refracting stones; then try the sunlight-card
method with genuine stones and with doublets and imitations until you
can tell every time whether you are dealing with singly or doubly
refracting material. When a stone of unknown identity comes along, try
the method on it and thus assign it as a first step to one or the other
class. Other tests will then be necessary to definitely place it.

DIFFERENCES IN REFRACTION DUE TO CRYSTAL FORM. The difference in
behavior toward light of the singly and doubly refracting minerals
depends upon the crystal structure of the mineral. All gems whose
crystals belong in the cubic system are singly refracting in all
directions: In the case of some other systems of crystals the material
may be singly refracting in one or in two directions, but doubly
refracting in other directions. No attention need be paid to these
complications, however, when using the sunlight-card method with a cut
stone, for in such a case the light in its course within the stone will
have cro