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Diamond Carat

A Beginner's Guide

By Thomas J Stevens - Last modified November 12, 2019

image of a diamond ring

What is a “Carat?”

A carat is a unit of measurement. It’s used to describe the weight of gems. Instead of using grams or milligrams, fractions of ounces, when weighing diamonds a very small measurement is used which is called a carat. “A one carat diamond” has a certain ring to it. The description of one carat doesn’t tell you anything about the quality or even the size of the diamond. Yup, that's right. One carat diamonds don't always look the same size. In this article you will find out why different diamonds of the same weight can look bigger than others.

First a little background on the name of this very small weight. a weigh scale with carob seeds as counterweight for a diamond The word “carat” is a word derived from carob, believe it or not. Gemstones were once weighed on balance scales, and gem dealers used carob seeds as counterweights, because it was believed that carob seeds all weigh the same. It turned out that carob seeds don’t all weigh the same, and carat weight varied widely across the globe. Luckily for us we have a much more reliable system for measuring the weight of gems and diamonds: the internationally agreed upon measurement of the carat. Carat weight was standardized to .200 g in the early 1900’s. Sometimes you may hear a jeweler talk about how many points a diamond is. This does not have anything to do with actual pointy points nor does it have anything to do with the diamond’s facets. Points are 1/100th of a carat. A 25 point (25pt) diamond is a .25ct or 1/4ct. 1 carat = 100 points. 1 carat = 200 milligrams or .200gm. It may seem that carat weight is the simplest and only truly objective factor of the 4C’s. Ok, while a diamond’s weight is not subjective, it’s easy to think that weight will correspond to a diamond’s apparent size. As you will soon discover this is not the case. It all depends on how a diamond is cut. What diamond weight does influence to a greater degree than the other of the 4C’s is price. Clarity, color, and cut all influence price, but none do so as much as the diamond’s weight.

Carat Weight Vs. Size

Two 1 carat diamonds can have different diameters. If one is cut into what is called a “deep cut” it will hide more weight in its pavilion. If the other one is a shallow cut it will look bigger from above. Neither of these are optimal cuts, as both shallow and deep cuts leak light through the sides and pavilion. Both types of cuts show less brilliance and life than stones that are well cut. While excess depth is undesirable in round diamonds, depth is a function of design with some fancy cut diamonds, which are cut to different specifications.

Carat Weight and Value

When all other factors are equal, carat weight will have the greatest impact on diamond prices. A 1.2ct G VS1 will cost more than a 1.0ct G VS1, but a 1.0ct D IF will cost more than a 1.5ct K SI1. Remember different combinations of the 4Cs affect price, but carat weight trumps all when comparing similar quality.

What Are “Magic” Weights?

“Magic” weights are the weights that are considered milestone weights. So we’re talking about weights like half a carat, one carat, one and a half carats, and such. It can even be 1.75 carats or other mid points. The price per carat increases the larger the size of the diamond since large diamonds are much harder to come by. The sharpest increases are usually at the “psychologically significant” points. You will often see prices jump to a greater degree between 0.49 and 0.50ct than between 0.47 and 0.48. The same applies to other price points like 1.00ct, 1.5 ct and such. As there’s is much higher demand for these round numbers, and 1ct weight is particularly popular, the price will naturally jump at these points. But it is important to be aware that a cutter will try to cut a diamond to reach these magic sizes. It is economically beneficial for a cutter to leave a little more weight on the crown, pavilion, and the girdle to push the stone to 1.00ct or more. Considering this fact it stands to reason that diamonds just under these magic weights will give you a little more for your money. These diamonds aren’t always easy to find, though, because, you guessed it, there’s a lot of savvy shoppers who know this tip.

As diamond size increases so does rarity and price. We're talking, of course about diamonds of the same quality. Prices increase at an accelerating rate with increases in size bringing the total price for larger stones to be much higher (on a cost per carat comparison). For example a 2.00ct diamond can end up being 4 times the total price of a 1.00 ct diamond of the same quality. That's a pretty big price increase. Evidently much more than double. The size relationship is a continuum, but one that has a number of psychological points along the line that are referred to as “magic marks” where there tend to be big price jumps. The 1.00ct mark is undoubtedly the best known. A 1.00ct diamond of a given quality might be more expensive than one would expect than the same quality weighing 0.98 with all else being equal. Many shoppers try to take advantage of this fact by looking specifically for diamonds just under these magic marks. Some of the most important ones are .50ct, .70, 1.00, 1.50, 2.00, 3.00, and 5.00.

Carat Weight Rounding

There’s a peculiar way that carat weights are rounded which works in the consumer’s favor. International carat weight rounding is done differently to the way rounding is usually done. When a jeweler says 1 carat, he must only refer to a diamond that is 0.999 carats or more. Diamond weights are rounded up from the third decimal point only when it is a nine.

Mounted Diamonds

Thinking of buying a mounted diamond? My suggestion is that you probably shouldn’t. Once a diamond gets set in a piece of jewelry it is not possible to weigh it with any sort of precision. Since it cannot be weighed the only way to estimated its weight is by measuring the outside dimensions of the stone. This is how its done for an appraisal. An appraisal is a report that places a value on a piece of jewelry. If you do have to get an appraisal a margin of error of about 5% is standard, and the report should be marked clearly that it is an estimate.

TCW: Total Carat Weight

Shopping channels love this one: TCW is the weight of all the diamonds on a piece. So pay attention to total carat weight when considering mounted diamonds in finished jewelry. A diamond ring may have a .50ct stone as its main stone with an additional .5ct in tiny diamonds. It’s perfectly legal to advertise it as as 1ctw or 1 tcw, which tells us that it is 1 carat total weight. Take note of the carat weight and check for that abbreviation. Be sure to ask the pertinent questions related to the carat weight of each stone, not just the total carat weight.

Ring Size and Carat Weight

Deciding how big you want your diamond? Wondering how a .5 carat diamond will look on a finger? One way is to check a diamond’s grading report for the size of the diamond. As two diamonds of the same weight can have slightly different measurements you really want to check the grading report for an accurate picture.

Carat Weight and Spread

The thickness of a diamond’s girdle if very thick will hide weight without adding to spread. If a diamond has a very thin girdle, the stone can have a wider spread. Since thinner girdles are more susceptible top chipping, GIA doesn’t consider a thin girdle to merit an excellent grade. Medium to slightly thick girdles are considered just right. Thicker girdles add extra weight but don’t add anything to the spread. Rather a thick girdle detracts from the diamond’s spread. Diamond carat size is so often thought to correspond to actual size. This of course can be very confusing. It’s one of those things that’s quite simple in concept but it takes a second to get it. Many people think that a larger carat size has to equal a bigger looking diamond. It doesn’t. Not automatically. And of course the price may seem good for a heavier diamond, but that is only because it is lacking in other areas. The other “Cs” all have an effect on the price of a diamond.
Takeaway: The “size” of a diamond refers to its weight. This is measured in carats. The actual size of the diamond’s diameter is shown on a grading report in millimeters.

Carot/Carob

The measurement unit of carats is actually an ancient unit of mass, rather than weight. Seeds of the carob tree were always used to weigh gold and, from the early 16th century, diamonds. Diamond traders used carat weight because they believed that all seeds were identical in size and weight. This turned out not to be the case. The difference in carob seed weights resulted in a wide variation in the weight of a carat until the early 20th century. In 1907, the General Conference on Weights and Measures came to an agreement on what the carat weight of one carat would be. It was decided that 200mg would equal 1 carat. It has since become internationally accepted. A 200 mg carat is now used everywhere when buying and selling diamonds. When weighed on a metric scale a 1.00 carat diamond weighs 200mg and a 5 carat diamond weighs 1000 mg (or 1 g.). Another method of measuring diamond weight is in “grains” where 1 carat equals 4 grains. The grain referred to is wheat.

The Size Difference Between a 1 Carat and 2 Carat

When you buy most commodities by weight, 2 units will usually give you twice as much as 1 unit and, although the same is true of diamonds, there is something akin to an iceberg principle at play.As an example, with modern round brilliant cut diamonds (although this similar principle applies to all cuts), a 1 carat diamond will be 6.4mm across the widest point, and a 2 carat will be 8.1mm across, not 12.8mm as might be expected. In other words, the 2 carat diamond is twice as heavy, even though it won’t appear to be twice as big. When setting the 2 carat diamond in a ring, it won’t appear to be much larger than the 1 carat diamond. You will only see a surface area increase of approximately 25-26% over the 1 carat diamond. These dimensions will vary depending on the type of cut and the corresponding crown area size. So, when talking about “size” in relation to diamonds, what is meant is ”weight”. It's not a reference to visible size. Visible size is a major factor in the appeal of a diamond, however the progression in size isn’t as pronounced as expected. This confusion between weight and visible size is the biggest cause of confusion, and often concern, among people browsing for an engagement ring or other jewelry which contains diamonds. You’re not getting less diamond, you’re just seeing less of what you pay for.

The Scale Of Diamond Size and Carat Weight

According to the Diamond Carat Size Guide for round brilliant diamonds, a 0.25ct diamond should be 4.1mm across, and a 0.5ct diamond 5.1mm, a clear difference of 1.0mm. However, a 1.25ct diamond should be 6.9mm, and a 1.5ct diamond (an identical 0.25ct increase) 7.4mm, a difference of just 0.5mm, or half of the difference for the smaller diamonds. This is despite broadly similar percentage differences in visible size for all the diamonds.

How Diamond Size Pricing Works

diamond price comparison chartBuying a big box of something is generally a better deal than buying two small boxes. Diamonds don't follow the same rule. This has to do with rarity: The law of supply and demand. One large diamond is worth far more than two smaller diamonds, even if their combined weight is the same. As the diamonds get bigger, they get progressively rarer, and so their price goes parabolic. A diamond that measures twice the size of another will cost many times the price. In order for the price to go up, all the other pricing variables will need to be kept the same.The size of the diamond as it appears on a hand won’t only depend on the carat size of the diamond. The size of the finger, the design of the ring, the cut of the diamond and the height of the setting will all impact the perceived size of the diamond. The prices shown here are roughly the prices for F color VS1 diamonds as of writing. This is based on all other areas being as close to equal as possible. There really are a lot less diamonds to compare once you reach the 5 carat price point or over, so it's no wonder that the price is so high. Another thing to be aware of is that sellers bundle the prices for diamonds in 0.50 carat intervals. What this means is that a 0.95ct diamond will not have a huge price increase over a 0.51ct, and yet be visibly quite close in size to a 1.00ct diamond. Many smart buyers take advantage of this information and choose carat size carefully. Of course, this technique works the same in reverse as well. If you do eventually come to sell the ring, don’t expect more than the 0.50ct price for your 0.95ct diamond.

Buying Tips to Get a Larger Diamond

Assuming that you want to have a budget in mind, here are the tips that you can use to ensure that you get the largest diamond.

Important Points:

Weight and visible size are but two elements to take into consideration when buying a diamond. Color, clarity and cut all affect the price of any individual diamond. Consider the visible diamond carat size, and how it may look on your finger. It may well be that the actual weight may not be as important if the diamond is a good looking one. Diamonds are sold by the carat (shown as ct.), which is actually a unit of weight, though most think of a carat in terms of size. The word “carat” comes from the the word “carob”. This was an ancient unit of measure for gem merchants. A carat is equal to exactly 0.2 grams. Carat weight is not to be confused with karat which is a measurement of the purity of gold. Two diamonds of equal carat weight may not share any other attributes in common and will therefore have completely different prices. As the carat size of a diamond increases, the diamond's price increases sharply. The larger the diamond, the rarer it is. Fewer than one in one million mined rough stones are large enough to produce a finished 1 carat diamond. So, as carat weight increases, you will typically pay more not only in total, but on a price-per-carat basis as well.

Diamond Carat Size

It is obvious that the price of a diamond increases with the carat weight. Sometimes exponentially so. But the actual size does not always relate to carat weight. The size relationship between diamonds of increasing carat weights is a little more subtle than you might expect. When carat weight increases from 1 to 3 carats you expect roughly triple the size, and this is actually the impression you get when looking at those two sizes side by side. But when you measure the the diameter you may be surprised to find that the increase is less than 50%. And the crown area only just doubles. This is something to remember when reviewing diamonds of any cut (shape). An increase in diameter will produce a larger increase in surface crown area as well as overall perceived size. A larger diamond looks much bigger than its mere measurements indicate when compared to a smaller diamond. When viewing diamonds check the measurements listed for each diamond to understand its size. The length and width given on a diamond grading report inform you of the precise measurement when the diamond is viewed when looking down on it. Another thing to be aware of is that the quality of the cut, or what's called the "make" also affects how much spread the diamond has. Two diamonds of the same cut shape may appear different in size even if they both weigh exactly the same. The proportions of the cut is what will make a difference. A narrow diameter is the result of a deep cut. A deeply cut diamond has a greater amount of its total weight that doesn’t contribute to the diamond’s diameter. All it does is make a taller diamond, which not only doesn’t look as big, it doesn’t return light as efficiently and this means less brilliance. See my article on diamond cut for more information about cut quality. These differences are sometimes small and may seem insignificant, but they can have a marked effect on the quality of the diamond. Diamond grading labs consider the depth of a cut to be an important factor when grading a stone. It is considered to be poor workmanship to hide weight in a stone, or to try to make the spread very wide and allow the brilliance to suffer. A well cut diamond can have a lower carat weight than a deeply cut diamond, while still having a larger diameter. This makes an excellent cut diamond appear larger in size than a poor cut (deep cut) diamond of the same weight.

Diamond Shapes Compared

Two diamonds that both have identical carat weight will not necessarily look equal in size if they are different types of cuts. As an example let’s take a 1 carat pear cut diamond. It might look larger than a 1 carat round brilliant due to differing geometry.

Size Estimation
The impression given by the longer shapes such as oval, marquise, pear and emerald appear to many people to look larger than round and square shapes, all else being equal: excellent cut, and identical weight.

Measurements
The measurements correspond to the shape shown above, and are typical for excellent cut diamonds of 1 carat weight.

Crown AreaUsing the Circumference or Perimeter
The area gives the true size of the diamond face up (as it would appear when set in a ring). Due to the difference in shapes, some fancy cuts might look bigger than a round brilliant. Some actually are bigger when viewed from above. Calculating the area of a stone will give you an indication as to whether it’s just an impression you are getting or if it is actually bigger in circumference/perimeter.

Remember, though, that a diamond that has been given anything less than an excellent grade may be due to its being a deep cut.

SHAPE (CUT):

Round Brilliant

round brilliant

Princess

princess-carat

Oval

oval-carat

Weight:

1.00 ct

1.00 ct

1.00 ct

Length
x
Width in mm

6.48
x
6.50

5.44
x
5.39

8.06
x
5.56

Area in mm2

33.06

29.32

35.19