It’s a self-contained instrument for measuring the refractive index of faceted gem stones, covering the full range from RI 1.270 to 3.094 and requiring no contact liquid.
The digital refractometer is calibrated to display Refractive Indices at the sodium “D” line at 589.3 nm as found in most reference charts. Due to the infrared light source, however, the test results may vary slightly with those found elsewhere.

The principle of operation utilizes the Freznels law and reflects a laser like beam of infrared radiation of the facet. Since only the top layers of atoms are scanned, it is essential that the facet placed over the examination disc is: Flat und scrupulously clean, well polished, dry and dust free, at least 3 mm (1/8 inch) in diameter and carefully centered on the examination disc.

You do not need to remove stones from most settings for testing. However, it is necessary that the facet be flat against the test disc, without interference from the setting. Please note that in some cases you need to measure more than one facet.