What is a refractometer and how is it related to diamond appraisals?

A refractometer is a device used to measure a gem's RI or reflection index. The refraction index (RI) measures the lustrous shine when light hits a certain gemstone. The gem sits atop a hemisphere that must have a higher RI index than the gemstone being tested. Thus a shadow is created and falls on a scale within the refractometer. This scale measures the RI index of the particular gemstone.
The refractometer is composed of a case on which the hemisphere is attached to the upper part. Light enters through the opening on one side, it proceeds to pass through the hemisphere and the gemstone, finally this creates a shadow which is measured on a scale within the refractometer and shown to the operator through a lens on the other side of the device. The value of the shadow on the scale determines the RI of a particular gemstone. For optimal optical contact to be achieved, a special solution must be added in between the hemisphere and the gemstone called an RI solution. In addition, the type of illumination used with the refractometer has to be very specific. Normal incandescent light that is polychromatic will disperse and create a wide, unclear line on the scale. Thus, monochromatic light is required, this will produce a single sharp line on the scale.

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