---
title: "Master stones: meaning in gemology"
description: "Master stones are reference diamonds of known color grade. A grader sets a diamond beside them to decide its D-to-Z letter."
url: https://digitalgemology.com/learn/glossary/master-stones
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Color and fluorescence

# Master stones

Also called master diamonds, master color comparison diamonds, master set

Master stones are reference diamonds of known color grade. A grader sets a diamond beside them to decide its D-to-Z letter.

## In more detail

Light and background change how a diamond looks, so labs don't grade color by eye in a showroom. GIA uses master diamonds in a standard viewing environment, with at least two independent graders. IGI also grades against master stones, with several gemologists.

## On a grading report

Behind every D-to-Z color grade on GIA and IGI reports.

## Related terms

- [Color grade](https://digitalgemology.com/learn/glossary/color-grade): The color grade is a letter from D to Z for the yellow or brown tint of a near-colorless diamond. D is colorless and Z is light yellow or brown.
- [Colorless (D–F)](https://digitalgemology.com/learn/glossary/colorless): Colorless is the top range of the D-to-Z scale: the grades D, E and F.
- [Near colorless (G–J)](https://digitalgemology.com/learn/glossary/near-colorless): Near colorless is the D-to-Z range from G to J.

## Learn more

- [The diamond color scale, D to Z: where you first see a tint](https://digitalgemology.com/learn/four-cs/diamond-color-scale-d-to-z)
- [The grades on a diamond report: color, clarity and cut](https://digitalgemology.com/learn/grading-reports/diamond-report-grades)

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