Negative reading

Negative reading: Testing a gemstone on a refractometer prism, when the stone has a greater refractive index than the measuring prism of the device, or higher than the refractive index of immersion liquid used for optical contact, the scale of refractometer remains dark and the only shadow edge that can be seen is that of immersion liquid, such stones or diamonds give a negative reading.

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Definition of Negative reading
http://m.alexandrite.net/viewpage.html?id=GG-099-00004

Testing a gemstone on a refractometer prism, when the stone has a greater refractive index than the measuring prism of the device, or higher than the refractive index of immersion liquid used for optical contact, the scale of refractometer remains dark and the only shadow edge that can be seen is that of immersion liquid, such stones or diamonds give a negative reading.

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Alexandrite Gemstone Encyclopedia & Jewelry Collector Guide
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Negative reading