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: 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: 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: 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: 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: A gem species is a mineral with one chemical composition and one crystal structure, such as corundum, beryl or quartz.