Carat
The size of a diamond has the biggest impact on its price. The metric carat, which equals 0.20 gram, is the standard unit of weight for diamonds and most other gems. If other factors are equal, the more a stone weighs, the more valuable it will be. Gemological laboratories measure carat weight when the diamond is loose (unmounted). While it is possible to estimate the weight of a mounted diamond, the lab uses ultrasensitive scales to achieve an exact weight, measured 3 decimal places, although the third decimal place is not usually mentioned at the retail level.
Here are several ways to express 1 carat:
- 1 ct.
- 200 milligrams
- 1/5 gram
- 100 points
- 4 grainer (not often used in retail environment)
Prices of diamonds are expressed in the trade as a price per carat. When we say that the Carat Weight has the biggest impact on the price of a diamond, not only is that true for the overall price of the stone, it is also true for the unit price per carat. That's why a 2 carat diamond is more than double the price of a 1 carat diamond.
Example:
Diamond "A" = 0.25 carats and costs $1,000 per carat. $1,000 x 0.25ct = $250/stone.
Diamond "B" = 0.50 carats and costs $1,250 per carat.
$1,250 x 0.50ct = $625/stone.
It is important to understand that when you compare two diamonds of the same shape, the one having a larger carat weight does not always translate to the larger looking stone. Dimensions play an important role in the diamond's appearance.
Large tables may reduce the brilliance of a Round Brilliant Cut, however they also do make a diamond appear larger than it is. Some people are more concerned with the "loss of beauty" while others are happy that their diamond seems "bigger".
Another aspect in how a diamond is cut can affect the size appearance. If the Diamond's "girdle" is too wide, this can make a stone with a higher carat weight look smaller with no benefit to its appearance. That is because the diameter of the diamond is widest at the girdle and "hiding" weight there does not significantly alter the behavior of the light entering the diamond.
Keep in mind that finger size is an important consideration as well. Small fingers will make a diamond appear larger.
It is very common for people to disregard the other C's in favour of getting the biggest possible stone they have budgeted for. Although SIZE DOES MATTER, we feel it is just as important for the quality of the stone to be great.
Some people may feel it is more impressive to wear a 2 carat diamond than a 1 carat diamond. But that's not necessarily true. A Ferrari may not be as big a car as an Oldsmobile, but most would consider it more impressive. What's most important if you are buying a Diamond for someone else, is that you know what their preference is.
How "big" is a carat?
Here's a simple trick to "understand" the meaning of a carat in realistic terms. Simply take a ruler and look at the table below. These are some approximate, sample carat-weight to diameter-widths for popular sizes.
0.25ct. ~ 4.1 mm  |
0.50 ct. ~ 5.2 mm  |
0.75 ct. ~ 5.9 mm  |
1.00 ct. ~ 6.5 mm  |
How much does "carat weight" affect cost?
The effect of all the different properties of diamond on cost is discussed in more detail in the cost section. For carat weight, let's take a typical Diamond through different carat weights to see by how much it increases in price.
A diamond of G color and SI1 clarity will be in one category of prices when it is between 0.50 - 0.69 carats. When you take that same quality Diamond and increase the size to the next price category, which is the 0.70 - 0.89 carat range, the price increase will be approximately $1,100 per carat (as of the date this was written). Increase to the 0.90 - 0.99 carat range, and the price increase will be approximately another $800 per carat. Increase to 1.00 - 1.49 carat range and the increase will be approximately another $800 per carat. If you increase to the 1.50 - 1.99 carat range, the price increase will be approximately $1,200 per carat.
Anecdotes from the Lab
Typically when a diamond comes into the lab, there is a special type of stone holder that holds the diamond.  However, when an enormous diamond comes into the lab to be graded, special measures need to be taken. Like grading the diamond using fingers to hold it in place!
Further Reading
"Total Carat Weight" and single-stone weight categories
Baguette Carat Weight Confusion
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DIAMONDS TUTORIAL NAVIGATOR
Preface:
Real Graders Don't Drink Kool-Aid
Lesson 1, The 5Cs:
Carat, Color, Clarity, Cut, Cost
Lesson 2, Shapes:
Round, Asscher, Cushion, Emerald, Heart,
Marquise, Oval, Pear, Princess, Radiant
Lesson 3, Articles:
How to Buy a Diamond, How to Sell Your Jewelry,
How to Read a Diamond Certificate,
The difference between a Certificate & an Appraisal,
What is the SI3?,
World's Largest Polished Diamonds,
World's Largest Rough Diamonds, Diamond Facts
Gem labs and the science, art and marketing of diamonds
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