Carat weight is often the first number people look at when choosing a diamond. But carat measures weight, not size. Two diamonds of the same carat weight can look quite different on the hand. Here is what carat really means and how to use it well.
What Is a Carat?
A carat is a unit of weight for gemstones. One carat equals 0.2 grams, and it is divided into 100 points. A 0.50 ct diamond is a "50-pointer," and a 1.00 ct diamond weighs exactly one carat.
Because carat is a weight, it does not tell you how big a diamond looks. Cut, depth and shape all change how large a stone appears from the top.
Carat Weight vs Diamond Size
Size is the stone's actual dimensions, usually measured across the top in millimeters. For a well-cut round diamond:
- 1.00 ct measures about 6.4 to 6.5 mm across.
- 1.50 ct measures about 7.3 to 7.4 mm across.
That is less than a millimeter of extra width for 50% more weight, and a noticeably higher price.
Shape matters too:
- Oval, marquise and pear shapes usually look larger than a round of the same weight.
- Asscher and princess cuts carry more weight in their depth, so they can look smaller.
Do Other Gemstones Weigh the Same?
No. Gemstones have different densities. Rubies and sapphires are denser than diamond, so a 1 ct ruby is smaller than a 1 ct diamond. Comparing carat weights across different gems does not compare their size.
Why Carat Weight Affects Price
Larger diamonds are much rarer than small ones, so price rises faster than weight. A 1.00 ct diamond usually costs well over twice as much as a 0.50 ct diamond of the same quality.
Prices also jump at popular weights such as 0.50 ct, 0.75 ct and 1.00 ct, because many buyers look for those numbers.
Tip: look just below the round numbers
- 0.90 to 0.95 ct instead of 1.00 ct
- 0.45 to 0.48 ct instead of 0.50 ct
The difference in size is very hard to see, but the difference in price can be real. For a stone to your exact specification, ask our Diamond Curation Service.
A Brief History of the Carat
The word carat is thought to come from the carob seed, which traders once used to weigh gems. In the early 1900s, the metric carat of 0.2 grams became the international standard, which is the system used today.
Carat Weight Is One Part of the Picture
Carat affects price and presence, but cut and shape decide how bright and how large a diamond looks. A well-cut stone in a flattering shape, such as an oval, can look larger than a heavier, poorly cut one.
At Tang&Co., lab-grown center stones of 1.00 ct and above come with an IGI report that states the exact weight and measurements. Browse our engagement rings, or find more answers in our FAQ.