The Digital Gem Refractometer is a nice optical instrument which will give a reading of the refractive index of a clean, flat, well-polished facet to reproducibility of 0.007RI index units. With covering the full range from 1.30 to 2.99RI, It is an ideal instrument to identify the gem stones. The importance is that it can test the gem stones without requiring ANY messy, damaging contact oil. Specification: RI Test Range:1.30 to 2.99RI Power Input: 9V Battery Work Tempreature: 25-28’C