Looking for a doubling effect of the culet through a crown bezel facet in a doubly refractive stone. The axis orientation (rough layout) will determine the direction of view.
As pictured the individual colors of the spectrum all bend differently according to their specific speed within the diamond.
A doubling effect will occur in doubly refractive gemstones. (Exaggerated for illustrative purposes.) Again, the view direction of the doubling will depend on the axis orientation of the stone.
In addition to the double refraction a moissanite commonly, although not always, contains needle-like inlusions arranged parallel to its pavilion.
Diamond is singly refractive — but some gemstones are doubly refractive such as alexandrite, aquamarine, citrine, kunzite, corundum (ruby/sapphire), spinel, tourmaline, moissanite etc. Light in doubly refractive gemstones travels at different speeds depending on the direction it enters the material. This is another method of distinguishing diamond from some of the imitations such as moissanite. Viewing the subject stone through a crown bezel facet, tilt it until the culet lines up in the center of the crown facet. With double refraction you will see two culets or at least a slight doubling, (overlapping) of the culet. Many times this doubling may be difficult to identify depending on the stone size, so don’t rely on that alone for identification.
Not only does the light bend as it enters the diamond but it also separates into the spectrum that we see in prisms or rainbows. This occurs because the different colors of the spectrum each travel at their own speed once they’ve hit the refractive material. Since they each travel at different speeds, they will each bend at a slightly different angle producing the dispersion we appreciate in a diamond. The slower the color in the light spectrum, the more time it has to bend while traveling through the diamond.