Similarly, ruby can at once be distinguished from even the finest garnet or ruby spinel, as the last two are singly refracting. So, too, are glass imitations of ruby and ruby doublets (which consist of glass and garnet). This test cannot injure the stone, it may be applied to mounted stones, and it is reliable. For stones of very deep color this test may fail for lack of sufficiently brilliant reflections. In such a case hold the card _beyond_ the stone and let the sunlight shine _through_ 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.