As a trained Metrologist (science of measurement) I would like to state a philosophical point about refractometers. The purpose of a refractometer is to measure the Refractive Index of a material and not to identify gems. It is your measurements, observations, separation efforts, and your Mind that Identifies the Gems. For example: You have a Red gem and a Green gem, both measure the same Refractive Index … Did the Refractometer Identify the Gems? Of course not, but the observation of Red or Green went towards the separation effort. Not many gems are above 1.81 so the first choice should be the critical angle refractometer utilizing RI fluid. I use a Gem-N-Eye Digital to confirm an OTL measurement and to see how high above 1.81 the gem under test might be. The standard refractometer utilizing RI fluid can give you information that the gem is doubly refractive or singly refractive, where the Digital units can’t readily provide that information. I hope this discussion was useful. 
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