Coloured Gemstone Carat Weight Calculator & Size Guide
Coloured gemstone carat weight calculator and guide using millimetre measurements, gemstone type and shape. Results are estimates only. Exact carat weight requires the stone to be unset and weighed loose on a calibrated gem scale.
Coloured Gemstone Carat Weight Estimator
For round or near round stones measure the diameter in two directions across the widest points. If you only have one diameter, repeat the same number for length and width.
If you leave depth blank, the calculator will assume a typical depth percentage for the chosen shape. Cabochons and antique cuts vary more than standard facetted stones, so results are always approximate.
Coloured gemstone carat weight explained
Carat is the unit used to describe gemstone weight. One carat equals 0.2 grams and is divided into 100 points, so a 0.50 carat stone is 50 points. It is easy to think of carat as size, but what you see on the hand depends on millimetre measurements, cut style and the density of the gemstone, not just the carat figure on a label.
Different gemstone families have different densities. A 7 × 5 mm sapphire will often weigh more than a 7 × 5 mm amethyst, even if they look similar in size. Emeralds usually weigh a little less again for the same face up size. The calculator uses typical densities for each gemstone type to reflect this, but real stones can still vary.
The most accurate way to know carat weight is to remove the stone from its setting and weigh it loose on a calibrated gem scale. This is not always practical, especially for antique jewellery, so an estimate based on careful measurements can still be useful when you are comparing pieces or trying to understand how a stone might feel on the hand.
How this coloured gemstone calculator works
The calculator uses three main measurements:
- Length across the longest point of the stone.
- Width at the widest point at a right angle to the length.
- Depth from table or dome to the base. If depth is unknown, the tool assumes a typical depth percentage for that shape.
For facetted stones the formula is based on a standard trade approach of Length × Width × Depth × shape factor. The shape factor reflects an average proportion for that cut style. Round brilliants, ovals, cushions and step cuts all use slightly different factors because they fill the volume in different ways.
The calculator then adjusts this shape factor according to the typical density for the gemstone type selected. Corundum gems such as ruby and sapphire are denser than quartz, so they weigh more for the same measured size. Quartz, emerald and beryl are less dense, so they weigh less. Garnets and spinels sit somewhere in between, with tourmaline also in the mid range. These adjustments keep the estimate closer to what you would expect in the real world.
Results are shown as a single estimated carat weight and a range. The range reflects normal variation in depth, girdle thickness and facet arrangement. Cabochons, rose cuts and antique cuts are especially individual, so the range is wider for these. The calculator is best used as a guide when you only have millimetre measurements rather than as a substitute for a professional valuation.
Examples – how size, species and cut change weight
To illustrate the difference density makes, the table below shows approximate weights for a typical 6 × 4 mm, 7 × 5 mm and 8 × 6 mm oval facetted stone with medium depth. These are rounded values that assume standard proportions and clean material.
| Face up size (mm) | Ruby or sapphire (corundum) |
Emerald (beryl) |
Quartz (amethyst, citrine, etc.) |
|---|---|---|---|
| 6 × 4 mm | ≈ 0.50 ct | ≈ 0.35 ct | ≈ 0.33 ct |
| 7 × 5 mm | ≈ 0.90 ct | ≈ 0.60 ct | ≈ 0.60 ct |
| 8 × 6 mm | ≈ 1.40 ct | ≈ 1.00 ct | ≈ 0.95 ct |
These numbers are typical rather than exact. Stones that are cut shallower or deeper, or that have unusually thick girdles, can sit outside these ranges. The calculator uses the same idea but allows you to plug in your own millimetre measurements and gemstone type for a more personalised estimate.
Cabochons behave differently again. Many cabochons are cut with a relatively shallow dome and a flat or slightly curved base, so they often weigh less than a facetted stone of the same outline. Heavily domed cabochons can do the opposite and hold more weight. This is why the calculator shows a wider range when you choose a cabochon shape.
Coloured gemstone carat and size FAQs
Is carat weight the same for all gemstones?
No. Carat is the same unit of weight for every gemstone, but different gems have different densities. A 1 carat sapphire will usually be smaller in millimetres than a 1 carat amethyst because sapphire is denser, so more weight is packed into a smaller volume.
How accurate is this coloured gemstone carat calculator?
It is designed as a guide based on common trade formulas, typical gemstone densities and average proportions for each shape. Results are shown with a range to reflect normal variation. The only way to know the exact carat weight is to have the stone unset and weighed loose on a calibrated gem scale or to rely on a trusted laboratory report.
Why do emeralds and other beryls often weigh less than sapphires for the same size?
Emeralds and other beryls usually have lower density than corundum. For the same millimetre measurements they occupy the same space but weigh less, so the carat weight is lower even though the face up size can look similar on the hand.
Can I use this calculator for cabochons?
Yes with care. Choose the cabochon option and measure length, width and the highest point of the dome. Cabochons vary more in dome height and base shape than standard facetted stones, so the estimate is more approximate and the range is wider.
Should I measure the gemstone in or out of the setting?
Measurements are always easier and more reliable on loose stones. Many vintage and antique pieces cannot be unset without risk, so measuring in the setting is sometimes the only option. In that case measure across the widest visible points without including the metal claws or bezel and treat the result as a guide.
Can I use this calculator for diamonds as well?
This page is tailored to coloured gemstones with their own density ranges. For diamonds it is better to use the dedicated diamond carat weight estimator and guide, which is based on proportions and formulas specific to diamond.
Does gemstone treatment change carat weight?
Most clarity and colour treatments do not significantly change carat weight. Very heavy surface-reaching fractures or cavities may be filled and can alter weight slightly, but for most jewellery pieces the difference is small. Treatment has a bigger effect on value and care than on weight.
What is the best way to know the exact carat weight of my coloured gemstone?
The most reliable method is to have the gemstone unset and weigh it loose on a calibrated gem scale. If the stone has an independent gemmological report, the carat weight on that report will already be based on precise weighing. Calculators and charts are helpful when you do not have a loose stone, but they remain estimates.
How does this relate to ring sizing?
Gemstone carat weight and ring size are separate. Carat tells you how much the stone weighs, while ring size tells you the inner diameter or circumference of the band. If you need help with ring sizing as well, you can use the Ring Size Calculator and Chart on the dedicated page or get in touch for guidance.