The first of the Four Cs: Carat weight

Diamonds are measured in carat weight, abbreviated to ‘ct’. One carat weighs 200 milligrams. The word ‘carat’ comes from the word ‘carob’, a bean that grows in the Mediterranean. In ancient times, if a diamond weighed the same as a carob bean, it was one carob, or one carat.

In the Far East, where carob trees do not grow, rice was used to measure the weight of a diamond. The standard used was four grains of rice – equivalent to what we now call one carat.

The majority of diamonds that are traded commercially are 1/3 of a carat. Carat weight is the easiest of the Four Cs to determine, because it is a precise measurement.

Weight in points

The weight of smaller diamonds is often expressed as points instead of carats. One carat equals 100 points. A 0.05 carat diamond, for example, weighs 5 points. A 0.75 carat diamond weighs 75 points or 3/4 carat.

Weight versus size

Carat weight tells you the size of a diamond as determined by its weight, but is not a factor that describes diamond quality. Purchasers usually prefer larger stones to smaller ones, so the carat weight does affect the value or price of a diamond. Larger diamonds naturally weigh more and are also rarer than the smaller ones, which means they command a higher price.

Useful tip

Beware when shopping for diamonds that are already set or mounted. If more than one diamond is used in a piece of jewelry, the tag on the jewelry will give the abbreviation ‘Ct TW’ or Carat Total Weight – it does not tell you the carat weight of each stone in the piece. You need to ask the jeweler for the total carat weight of the largest diamond in the piece to truly understand the value of the diamonds you are buying.

You may want to understand carat weight in terms of size. Here's a simple way. These are some approximate, sample carat-weight to diameter-widths for popular sizes.

0.25 ct. – 4.1 mm

0.50 ct. – 5.2 mm

0.75 ct. – 5.9 mm

1.00 ct. – 6.5 mm

Carat weight is only one of the Four Cs that you should evaluate before you buy a diamond. We recommend that you take some time to understand more diamond basics before you start shopping.

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