The refractometer is considered by some to be the most important of all gem-testing instruments. Determine refractive index (RI), identify optic character (SR / DR), and measure birefringence (Bire).
- Most refractometers have six main parts:
- Hemicylinder to set the stone on.
- Scale, with RIs marked on it.
- Viewing lens to see the scale
- the removable magnifier for scale
- polarizing filter for measuring birefringence of DR stones.
- light portal in the back
Use both white and monochromatic light. For white light, a Polaris cope will do. The utility lamp simulates monochromatic sodium light with a yellow filter.
Contact liquid ensures an optical connection between a gem and the hemicylinder. The liquid's RI must be lower than the hemicylinder's but higher than the gem's. RI liquid near 1.78 is best.
There are two basic methods for taking RI readings; one for faceted stones, and one for cabochons and carvings.
Always take a flat facet reading if you can; they are generally more accurate.