Gem Lab
Polariscope
Between two crossed polarising filters a stone reveals how it refracts light. A simple instrument with meaningful results.
The polariscope is among the simplest instruments in the laboratory, yet it provides important clues. It shows whether a material is singly or doubly refractive and helps to recognise certain structures.
Design
The instrument consists of a light source, a lower polarising filter, the polariser, and an upper filter, the analyser. When the two are crossed, the field of view appears dark. The stone is placed between them and rotated.
What is observed
- Singly refractive materials such as spinel, garnet or glass stay dark when rotated.
- Doubly refractive materials such as corundum, beryl or tourmaline turn light four times and dark four times during a full rotation.
- Anomalous double refraction appears as patchy or banded brightening. It occurs in some singly refractive materials because of internal strain, for example in glass or synthetic spinel.
- Aggregates, materials made of many small crystals such as jade, stay evenly light when rotated.
The conoscope
With an additional lens, a so-called conoscope sphere, interference figures can be made visible. They show whether a stone is optically uniaxial or biaxial. This narrows down the identification further but requires some practice.
The limits
The polariscope only answers how a stone refracts light, not exactly which material it is. Heavily included, very dark or small stones are difficult to assess. The result is therefore always combined with other measurements, such as the refractometer.
In the course of an examination
The polariscope is usually used early because it is quick and does not stress the stone. If it shows a doubly refractive stone although the appearance suggests spinel, that is a clear reason for further tests.
In brief
The polariscope shows whether a stone is singly or doubly refractive and reveals internal strain. It is quick and gentle and is therefore used early. Its results are always combined with further measurements.
At NOBILITH
The laboratory reports on our stones rest on a combination of such standard instruments with more advanced methods. In the Academy we explain the instruments so that the statements in a report become understandable.
Sources
- GIA: Gem Encyclopedia, polariscope
- 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 LabRefractometerThe refractometer measures how strongly a stone bends light. This allows many materials to be narrowed down quickly – within clear limits.Read more
Updated 2026-10-06