Gemstone size chart: carat to mm
By HeyGema™ · Updated
In short
Gems of the same weight are not the same size. A 1 carat round sapphire is about 6.2 mm across, a 1 carat diamond 6.5 mm and a 1 carat emerald 7.0 mm.
Round gemstone size by carat
Face-up diameter in mm of a well-cut round stone. Density is in g/cm3.
- Diamond
- Density: 3.52
- 0.5 ct: 5.1 mm
- 1 ct: 6.5 mm
- 2 ct: 8.1 mm
- 3 ct: 9.3 mm
- Moissanite
- Density: 3.22
- 0.5 ct: 5.3 mm
- 1 ct: 6.6 mm
- 2 ct: 8.4 mm
- 3 ct: 9.6 mm
- Sapphire
- Density: 4.00
- 0.5 ct: 4.9 mm
- 1 ct: 6.2 mm
- 2 ct: 7.8 mm
- 3 ct: 8.9 mm
- Ruby
- Density: 4.00
- 0.5 ct: 4.9 mm
- 1 ct: 6.2 mm
- 2 ct: 7.8 mm
- 3 ct: 8.9 mm
- Emerald
- Density: 2.72
- 0.5 ct: 5.6 mm
- 1 ct: 7.0 mm
- 2 ct: 8.9 mm
- 3 ct: 10.1 mm
- Aquamarine
- Density: 2.70
- 0.5 ct: 5.6 mm
- 1 ct: 7.0 mm
- 2 ct: 8.9 mm
- 3 ct: 10.2 mm
- Morganite
- Density: 2.80
- 0.5 ct: 5.5 mm
- 1 ct: 7.0 mm
- 2 ct: 8.8 mm
- 3 ct: 10.0 mm
- Tanzanite
- Density: 3.35
- 0.5 ct: 5.2 mm
- 1 ct: 6.6 mm
- 2 ct: 8.3 mm
- 3 ct: 9.5 mm
- Amethyst
- Density: 2.65
- 0.5 ct: 5.6 mm
- 1 ct: 7.1 mm
- 2 ct: 8.9 mm
- 3 ct: 10.2 mm
- Citrine
- Density: 2.65
- 0.5 ct: 5.6 mm
- 1 ct: 7.1 mm
- 2 ct: 8.9 mm
- 3 ct: 10.2 mm
- Blue topaz
- Density: 3.53
- 0.5 ct: 5.1 mm
- 1 ct: 6.4 mm
- 2 ct: 8.1 mm
- 3 ct: 9.3 mm
- Peridot
- Density: 3.34
- 0.5 ct: 5.2 mm
- 1 ct: 6.6 mm
- 2 ct: 8.3 mm
- 3 ct: 9.5 mm
- Garnet
- Density: 3.90
- 0.5 ct: 4.9 mm
- 1 ct: 6.2 mm
- 2 ct: 7.9 mm
- 3 ct: 9.0 mm
- Tsavorite
- Density: 3.61
- 0.5 ct: 5.1 mm
- 1 ct: 6.4 mm
- 2 ct: 8.1 mm
- 3 ct: 9.2 mm
- Spinel
- Density: 3.60
- 0.5 ct: 5.1 mm
- 1 ct: 6.4 mm
- 2 ct: 8.1 mm
- 3 ct: 9.2 mm
- Tourmaline
- Density: 3.06
- 0.5 ct: 5.4 mm
- 1 ct: 6.8 mm
- 2 ct: 8.5 mm
- 3 ct: 9.7 mm
Why gems of the same carat are different sizes
A carat is a weight (0.2 grams). Dense gems such as sapphire, ruby and garnet pack more weight into less space, so they face up smaller. Light gems such as emerald, aquamarine and amethyst face up bigger.
Questions
A 1 carat round tsavorite measures about 6.4 mm across.
A 1 carat round sapphire measures about 6.2 mm across, smaller than a 1 carat diamond at 6.5 mm.
A 1 carat round emerald measures about 7.0 mm across. An emerald cut stone is longer than it is wide, so ask for its length and width.
Moissanite is lighter than diamond, so a 1 carat moissanite measures about 6.6 mm across. It is often sold by its size in mm instead of its weight.