Gem Lab
Refractometer
The refractometer measures how strongly a stone bends light. This allows many materials to be narrowed down quickly – within clear limits.
When light enters a gemstone it is refracted, meaning its direction changes. How strongly depends on the material: this is the refractive index. The refractometer measures this value and is one of the basic instruments in any gemmological laboratory.
How it works
The stone is placed with a polished facet on a glass prism with a high refractive index. A contact liquid provides optical contact. Light, usually yellow sodium light or an equivalent filter, passes through the prism. At the boundary between prism and stone a shadow edge appears, whose position is read on a scale.
What can be read
- Refractive index: corundum at about 1.76–1.77, beryl at about 1.57–1.58, tourmaline at about 1.62–1.64
- Birefringence: the difference between highest and lowest reading in doubly refractive stones
- Optic character: uniaxial or biaxial, positive or negative, by rotating the stone and a polarising filter
These values narrow down the possible materials considerably. A stone that looks like a sapphire but reads 1.54 is not corundum.
The limits
The measurement requires a flat, well-polished surface. On cabochons only a spot reading with lower accuracy is possible. The contact liquid usually limits the range to about 1.79; diamond and other highly refractive materials lie above it. Natural and synthetic stones of the same species have practically the same values – the refractometer cannot tell them apart.
Safety
The usual contact liquid contains toxic components and is used sparingly and carefully. Porous or filled stones can absorb liquid and are handled with corresponding care.
In brief
The refractometer measures refractive index and birefringence and so narrows down many materials quickly. It needs a polished surface and only reads up to about 1.79. It cannot distinguish natural from synthetic stones of the same species.
At NOBILITH
The refractometer is part of the standard examination on which the laboratory reports for our stones are based. Not every report states the measured refractive index, but identification regularly relies on it.
Sources
- GIA: Gem Encyclopedia, refractometer
- Read, P.: Gemmology
Read more
- Laboratory reportsUnderstanding a laboratory reportA laboratory report is a gemstone's most important document. Which information it contains, how to read it and what it does not say.Read more
- KnowledgeWhy a laboratory may say inconclusiveSometimes a question cannot be answered responsibly with the available data. What “inconclusive” means – and what it does not.Read more
- Gem LabGemmological microscopeThe most important instrument in the laboratory: under the microscope, inclusions, growth features and signs of treatment become visible.Read more
- Gem LabPolariscopeBetween two crossed polarising filters a stone reveals how it refracts light. A simple instrument with meaningful results.Read more
Updated 2026-10-06