---
title: "Refractometer: meaning in gemology"
description: "A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of…"
url: https://digitalgemology.com/learn/glossary/refractometer
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Gem testing

# Refractometer

Also called gem refractometer, critical angle refractometer

A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of higher index.

## In more detail

The reading is where a shadow edge falls on the scale. A standard instrument has a glass of about 1.88 and 1.81 liquid, so it cannot read stones above 1.81. A doubly refractive stone gives two edges; their gap is the birefringence. Real readings have a tolerance of about 0.005 on a well-polished facet.

## How we measure it

Our Refractometer models a standard instrument: glass 1.88, liquid 1.81 and yellow light at 589 nm. The Fresnel equations, for both polarizations, light each point of the scale. It is a teaching model, not a test of a real stone.

## Related terms

- [Shadow edge](https://digitalgemology.com/learn/glossary/shadow-edge): The shadow edge is the line between the dark and the bright part of a refractometer scale. It falls at the refractive index of the stone.
- [Over the limit (OTL)](https://digitalgemology.com/learn/glossary/over-the-limit): Over the limit (OTL) is the refractometer reading of a stone whose refractive index is higher than the contact liquid's. The scale stays dark down to the liquid line, so there is no number to read.
- [Contact liquid](https://digitalgemology.com/learn/glossary/contact-liquid): Contact liquid is the drop of high-index liquid between a stone and the glass of a refractometer. It fills the air gap so that light can pass from the glass into the stone.
- [Refractive index](https://digitalgemology.com/learn/glossary/refractive-index): Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.
- [Birefringence](https://digitalgemology.com/learn/glossary/birefringence): Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.

## Learn more

- [Refractometer](https://digitalgemology.com/tools/refractometer)
- [Refractometer](https://digitalgemology.com/about/how-we-measure/refractometer)

[All 328 terms](https://digitalgemology.com/learn/glossary)