A refractometer (analog) is one of the gemmologists' most important tools (digital models are also available). It is used to quickly and non-destructively measure the refractive index (RI) of a gemstone. This value helps identify the mineral of the stone and reveal whether it is a real stone, a synthetic or an imitation.
A brief guide to how the instrument works and is used:
Measuring principle: The instrument measures the so-called critical angle of light that is refracted when it passes from the cylinder (a prism) into the tested gemstone.
Contact liquid: In order for the light to be transmitted correctly between the prism and the stone, a special contact liquid is required. This liquid has a very high refractive index (often (n approx 1.80)).
Reading: When you look through the eyepiece, you see a light and a dark zone. The boundary between them is called the shadow line and points directly to the exact refractive index of the stone on a built-in scale.
Birefringence: Many gemstones split the light into two beams. By turning a polarizing filter in front of the eyepiece, the gemologist can see how the shadow line moves and thus determine the stone's birefringence, which provides an even more reliable identification.
Measuring range and limitations: Most analog instruments measure refractive indices between (1.30 text{ and } 1.81). This means that stones with extremely high refractive indices, such as diamond (about (2.42)), cannot be measured. The instrument also requires a flat, well-polished facet surface to give an accurate reading.
