Amethyst Carat to mm Size Chart
Quick answer
A 1 carat round amethyst is about 7.0 mm across and a 2 carat about 8.8 mm. A 1 ct oval is about 8.5 x 6.1 mm. With a specific gravity of 2.65, amethyst looks larger than a diamond of the same carat weight.
Typical depth and outline assumed; deep-cut stones face up smaller. Data verified .
Amethyst carat to mm calculator
Amethyst size by carat weight
Amethyst is purple quartz. Quartz is one of the lightest common gem materials, so amethyst gives a big look for the carat weight. The same chart works for citrine, smoky quartz, rose quartz and clear quartz, which share the density.
Because large clean amethyst is widely available, jewelry often uses stones of several carats. Many amethysts are cut to calibrated millimetre sizes, so sellers and buyers often think in millimetres first.
| Carats | Round | Oval (1.4 ratio) | Cushion | Diamond round, same weight |
|---|---|---|---|---|
| 0.25 ct | 4.4 mm | 5.3 x 3.8 mm | 4.2 mm | 4.1 mm |
| 0.50 ct | 5.6 mm | 6.7 x 4.8 mm | 5.3 mm | 5.1 mm |
| 0.75 ct | 6.4 mm | 7.7 x 5.5 mm | 6.1 mm | 5.9 mm |
| 1.00 ct | 7.0 mm | 8.5 x 6.1 mm | 6.7 mm | 6.5 mm |
| 1.50 ct | 8.0 mm | 9.7 x 6.9 mm | 7.7 mm | 7.4 mm |
| 2.00 ct | 8.8 mm | 10.7 x 7.6 mm | 8.4 mm | 8.1 mm |
| 3.00 ct | 10.1 mm | 12.2 x 8.7 mm | 9.7 mm | 9.3 mm |
| 4.00 ct | 11.1 mm | 13.4 x 9.6 mm | 10.6 mm | 10.2 mm |
| 5.00 ct | 12.0 mm | 14.5 x 10.3 mm | 11.4 mm | 11.0 mm |
Generated by our formula engine at typical depth: 61% of width for rounds and ovals, 70% for cushions. Round sizes carry a range of about plus or minus 2% that comes from the spread in published factors and in amethyst density (2.63 to 2.66).
Amethyst compared with diamond
Amethyst has a specific gravity of 2.65 against 3.52 for diamond. At the same carat weight, a amethyst is larger than a diamond, by about 10% in diameter. Put the other way round, a amethyst cut to the size of a 1 ct diamond weighs about 0.75 ct.
Use the diamond equivalent weight calculator to compare amethyst with diamond, moissanite and other gems side by side, or the mm to carat calculator if you have a stone to measure.
Common amethyst shapes
Every shape is common: ovals, cushions, rounds, pears, emerald cuts and fancy shapes. Calibrated sizes such as 8 x 6 mm or 10 x 8 mm ovals are standard in mass-produced settings.
Buying notes
- Order by millimetres if your setting has a fixed seat, and use carats only to compare value.
- Amethyst is 7 on the Mohs scale, durable for most jewelry but softer than sapphire or diamond.
- Strong light and heat can fade some amethyst color over time. Store pieces away from sunny windowsills.
- Big stones are affordable, so price per carat rises more gently with size than it does for rare gems.
Amethyst at a glance: specific gravity 2.65 (range 2.63 to 2.66), Mohs hardness 7.
Other gemstone size charts
- Sapphire Carat to mm Chart
- Ruby Carat to mm Chart
- Emerald Carat to mm Chart
- Topaz Carat to mm Chart
- Garnet Carat to mm Chart
- Aquamarine Carat to mm Chart
- Spinel Carat to mm Chart
- Cubic Zirconia Carat to mm Chart
- Diamond size chart in mm
- Gemstone specific gravity chart
Related questions
How many mm is a 1 carat amethyst?
About 7.0 mm for a round at typical depth, likely 6.9 to 7.2 mm. A 1 ct oval amethyst is about 8.5 x 6.0 mm.
How big is a 2 carat amethyst?
A 2 ct round amethyst is about 8.8 mm, and a 2 ct oval amethyst with a 1.4 ratio about 10.7 x 7.6 mm. At 3 ct, a round amethyst reaches about 10.1 mm.
Is a 1 carat amethyst bigger than a 1 carat diamond?
A 1 ct amethyst is larger than a 1 ct diamond: 7.0 mm against 6.5 mm round, about 10% larger across. In volume it matches a 1.33 ct diamond.
How much does a amethyst the size of a 1 carat diamond weigh?
A amethyst cut to that size weighs about 0.75 ct, because weight at the same size follows density (SG 2.65 for amethyst against 3.52 for diamond).
Does this chart work for citrine?
Yes. Citrine is also quartz with the same density, so sizes are the same.
How many carats is a 10 x 8 mm oval amethyst?
It depends on depth. Enter the measurements in the mm to carat calculator with amethyst selected for an estimate with a range.
Sources and method
Weight estimates use the colored-stone shape factors published by the International Gem Society and the Dendritics standard formula, and the diamond trade constants and length to width ratio tables published in jewelry trade references (Esslinger, Omni, Octonus). Specific gravity values are cross-checked against the International Gem Society density tables, the USGS Natural Gemstones report and GemSelect.
Every number on this page is generated by the same formula engine as the calculators. Read the full method, constants and limits and the editorial policy. Data last verified 2026-11-04.
Estimates only. This is not an appraisal, grading report or valuation. A calibrated scale or a lab report is the only real weight.