---
title: "Specific gravity: meaning in gemology"
description: "Specific gravity compares a stone's weight with the weight of the same volume of water. It decides how much a stone of a given size weighs."
url: https://digitalgemology.com/learn/glossary/specific-gravity
image: https://digitalgemology.com/img/og-digital-gemology.png?v=d224fb01
---

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[Glossary](https://digitalgemology.com/learn/glossary) · Gemstone properties

# Specific gravity

Also called SG, density

Specific gravity compares a stone's weight with the weight of the same volume of water. It decides how much a stone of a given size weighs.

## In more detail

Cubic zirconia is 5.6 to 6.0, ruby and sapphire 4.00, diamond 3.52, moissanite 3.22, emerald 2.72 and amethyst 2.65. At the same size, a ruby weighs about 1.14 times as much as a diamond. A moissanite weighs about 0.91 times as much. So carat weight alone does not tell you the size.

## How we measure it

Every weight on our cut pages comes from the model's measured volume and the stone's specific gravity. For cubic zirconia we use 5.8.

## Related terms

- [Weight from volume](https://digitalgemology.com/learn/glossary/weight-from-volume): Weight from volume is how we calculate a model's weight: m = V × ρ ÷ 200 carats. V is the volume in mm³ and ρ the specific gravity in g/cm³.
- [Face-up size](https://digitalgemology.com/learn/glossary/face-up-size): Face-up size is how large a stone looks from above for its weight. Two stones of the same carat weight can differ in face-up size because of their depth and shape.
- [Heavy liquid](https://digitalgemology.com/learn/glossary/heavy-liquid): A heavy liquid is a dense liquid that sorts stones by specific gravity. A lighter stone floats in it, and a denser one sinks.
- [Carat weight](https://digitalgemology.com/learn/glossary/carat-weight): Carat weight is the weight of a gemstone. One carat is 200 milligrams, a fifth of a gram, and it has 100 points.
- [Gem species](https://digitalgemology.com/learn/glossary/gem-species): A gem species is a mineral with one chemical composition and one crystal structure, such as corundum, beryl or quartz.

## Learn more

- [Gemstone constants](https://digitalgemology.com/about/how-we-measure/gemstone-constants)
- [Moissanite vs diamond: same size, different carat weight](https://digitalgemology.com/learn/diamond-vs-moissanite/moissanite-vs-diamond-carat-size)
- [Ruby 3D models and cutting files](https://digitalgemology.com/gemstones/ruby)

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