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Carat Weight + Rapaport Pricing <The Ultimate Diamond Buying Guide>

*Do note that our writings are based on our personal opinion and only serves as a reference or guide.

In this article, we will explain some cost saving tips by avoiding flat carat weights and how to maximise your diamond's apparent size.

The Carat Weight

The carat weight, which is one of the 4C's of a diamond, is a unit of measurement for gemstones, not just for diamonds.

1 carat equals to 0.2 grams.

As jewellers, we can usually estimate the size of a diamond based on the size of it that we see on someone's hand.

Carat weight does not always determine how big the diamond appears to our eye, as the proportions of a diamond could make it look smaller (face up) than the weight it suggests. For example, a shallow cut diamond could look much bigger on the face up. Hence for each diamond weight, there is an approximate diameter size it should have.

1.00 Carat Diamond has a Recommended Diameter of 6.5mm

If a 1 carat diamond, has an average girdle diameter smaller than 6.5mm, it is known as a overweight diamond. Generally overweight diamonds are acceptable. As long as they are not overweight by more than 9%, they are still graded as Excellent cut in GIA. If the overweight percentage is huge, it would mean that your diamond looks small but has a heavy carat weight. This means you are paying more for a diamond that looks relatively smaller.

For a 1 carat diamond with an average girdle diamond of 6.1mm.

Overweight Percentage = (Recommended Diameter - Average Girdle Diameter)/Recommended Diameter x 100%

= (6.5mm - 6.1mm)/6.5mm x 100%

= 6.15 %

For difference carat weights, you can refer to this approximation chart, where (RD) refers to recommended diameter and (CT.) refers to carat weight.

Personally I have a preference for diamond which are underweight or not overweight by more than 3%, hence I would use these calculations. But rest assured that if you buy an GIA Excellent Cut diamond, the overweight percentage will be less than 9%. It is up to you to decide on how stringent you want to be with your diamonds.

*Tip - Settings with more prongs/claws can make a diamond look bigger. The 6 prong setting is our favourite.

Avoid Flat Carat Weights

Flat carat weights refer to diamond weights with a zero at the back. For example 0.50, 0.70, 0.90 etc. The saying to avoid these flat carat weight diamonds because you will be paying more has truth in it.

In the B2B market where retailers buy diamonds from the international diamond trading network RapNet, diamonds are sold based on a price chart also known as the Rapaport Price List. And in this price list, diamonds are catogarised based on their carat weight ranges. Here is an actual price list publish by Rapaport on 15 July 2016.

In the black header, we can see the range 0.90 - 0.99 ct is given a specific price for a D IF diamond at 14500 USD per carat. Now we compare it to the next table 1.00 - 1.49 ct D IF diamond which is priced at 21600 USD per carat.

If we were to buy a 0.99 ct diamond, the price would come out to be = 14500 x 0.99 = 14355 USD

Now if we buy a 1.00 ct diamond, the price is = 21600 x 1.00 = 21600 USD

Visually there is no difference between a 0.99 ct diamond and 1.00 ct diamond. They look the same. Our eyes can't pick out a 0.01ct difference, only weighing scales can. But notice how much more you would be paying?

Of course if you are buying the diamond as a gift, it is always nicer sounding to say that it is a 1 carat and above. However, it is also a personal cost saving preference, or based on your understanding of the recipient.

This flat carat weights hold true for most diamond ranges, 0.50, 0.70, 0.90 with the same concept.

But 0.99 ct vs. 1.00ct may be an exception depending on your jeweller. While theoretically it follows the chart, here is why the exact weight 0.99 ct vs 1.00 is a special exception that may not always be true.

As explained in my Introduction to Diamonds, this is because all diamond cutters would try to achieve the 1.00 ct weight instead of 0.99, making 0.99 much lesser in the market. A 0.99ct diamond is usually only made upon a cutter’s mistake. Jewellers would also only stock 1.00 above diamonds instead of exactly 0.99. Hence to satisfy some customers’ request of wanting exactly a 0.99ct diamond, we have heard of jewellers who would shave away 0.01ct off from a 1.00ct diamond to achieve the desired weight and send it for regrading at GIA. Unless they specially source from suppliers, this shaving process usually results in a premium for the service and regrading costs, unlike what many consumers would think in terms of cost savings.

So while you can do it for other carat weight ranges, this specific weight range is an exception that sometimes you may get lucky, sometimes maybe not. There is generally a huge price jump for 1.00 above diamonds, so it really depends on what the jeweller has to offer. A 0.97 or 0.98 on the other hand will be a good cost saving choice.

Head back to our Ultimate Diamond Buying Guide to learn about the other components of diamond grading.

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