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.