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.

See Alexandrite Tsarstone collectors guide, 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.) (as of Apr. 03, 2026, 15:36 GMT).