---
title: "Over the limit (OTL): meaning in gemology"
description: "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…"
url: https://digitalgemology.com/learn/glossary/over-the-limit
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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

# Over the limit (OTL)

Also called OTL

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.

## In more detail

Diamond, moissanite and cubic zirconia all read over the limit with 1.81 liquid. So a standard refractometer cannot tell these three apart. Moissanite's double refraction does not show on it either.

## How we measure it

Our Refractometer shows the OTL view: the scale stays dark down to a faint liquid line at 1.81. The liquid's own critical angle in a glass of 1.88 is 74.3°.

## 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.
- [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.
- [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.
- [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)
- [Diamond testers and moissanite: what the instruments say](https://digitalgemology.com/learn/diamond-vs-moissanite/diamond-tester-and-moissanite)

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