Refractometer

The instrument in general use for the measurement of refractive index is the refractometer. The refractometer determines the refractive index of a gemstone by using the principle of critical angle of total internal reflection, which is used to measure the refractive indices of various substances.

General information

- Refractive Index (RI) measurement is the most useful and accurate means of testing gemstones.
- It is convenient, non-destructive and relatively quick to carry out providing useful information.

Requirements

- Optical contact.
- Clean, polished surface.
- Contact liquid.
- Refractive index (RI) of the stone under test should be within the range of the instrument.

Limits

- Upper limit of the refractometer is determined by the refractive index of the prism as well as the refractive index of the contact fluid / medium.
- In a normal commercial refractometer, this is 1.81 (some RI liquids may have a limit of 1.79).

Results

- Flat surface readings: single refractive (SR) / double refractive (DR), birefringence, optic character (uniaxial or biaxial), and optic sign (positive or negative).
- Curved surface readings (distant vision method): Mean refractive index only.