---
title: "Shadow edge: meaning in gemology"
description: "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."
url: https://digitalgemology.com/learn/glossary/shadow-edge
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Shadow edge

Also called refractometer reading

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.

## In more detail

Above the edge the scale is dark, and below it bright. A doubly refractive stone shows two edges, with a half-bright band between them. A singly refractive stone shows one edge, which stays in place as the stone turns.

## How we measure it

Our Refractometer places each edge exactly at the published index. Amethyst, for example, shows edges at 1.544 and 1.553.

## Related terms

- [Refractometer](https://digitalgemology.com/learn/glossary/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.
- [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.
- [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.
- [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.

## Learn more

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

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