Doubly refractive stones inherently have two refractive/reflective values. While optical refractometers are capable of
observing both refractive values, electronic refractometers/reflectivity meters only observe a single value which is between
the two actual values.
The advantage of electronic refractometers and electronic reflectivity meters, though, is their ability to take measurements
of refractive indexes up to a value of 3.00, whereas optical refractometers can only measure up to the refractive index of the
oil used, the highest available being a value of 1.81. This allows digital refractometers to identify moissanites, diamonds, and
other stones of high refractive indices.