Diamond Carat Weight

The public (that’s you) expresses diamond carat weight in percentages of carat weight, such as quarter carat, half carat, three quarter carat, one carat, etc.  Not surprisingly, people also tend to think of the value of your ring, or how much you spent on it by your answer of what it weighs.  The reality however is that other factors such as diamond color, diamond clarity and diamond cut quality also have a significant effect upon the price of a diamond and thus it is common for smaller diamonds to cost more than larger ones.

Within the diamond industry there are several ways of expressing diamond carat weight. A one carat diamond weighs 100 points, just like there are 100 pennies in a dollar, and would be expressed in written form as 1.00 carats. The term “points” does not refer to the number of facet junctures that are created by the facet structure of a diamond, it is a representation of weight which originated in Egypt when carob seeds were used to determine the weight of a gem stone. When describing a half carat diamond weighing 0.52 carats, a store owner might refer to it as a “52 pointer” instead of saying it weighs “zero (point) five two carats)”.

In the old days, diamond wholesalers frequently referred to diamonds that weighed about one-quarter carat, or its multiples, as being so many grains. A grain roughly equals twenty five points or one-quarter carats, expressed as 0.25 carats. A 0.74 carat diamond, for example, would be referred to as a three-grainer. A 1.25 carat diamond would be called a five grainer and so on, today the stone would be described as “a carat twenty five” or a “carat and a quarter” but a few of the old timers still talk amongst themselves in terms of grains just to see if us young pups are up to speed.

Keeping It Legal:

The Federal Trade Commission has set forth guidelines concerning the proper representation of carat weight to the public. The diamond’s actual weight must fall within two points (0.02 carats) of the fractional representation. For instance, a diamond represented as weighing one-carat should fall within a range of 0.98 to 1.02 carats. A half-carat diamond should weigh between 0.48 and 0.52 carats, and so on.  A diamond weighing 0.72 carats should not be represented by a person within the industry as being a three-quarter carat.  The fact of the matter is that your significant other can call her sixty-eight pointer a three-quarter carat, and her girlfriends will just have to take her word for it…  We, on the other hand, can’t get away with that and have to express the weight of the diamond in correct legal terms.

Truth In Advertising Diamond Carat Weight:

When buying diamond jewelry advertised as having “a carat total weight” you should make sure that you’re getting what you pay for…  A pair of one-carat total weight diamond earrings or a one-carat total weight cluster style ring should have a combined total diamond weight that falls somewhere between 0.98 and 1.02 carats…  If the combined total weight is actually 0.85 carats, then you’re not buying a carat total weight and you should realize the difference.  On the other hand, if the one-carat total weight diamond ring that you’re looking at actually contains a total weight of 1.10 carats then the other eight points are a bonus for you and you should consider yourself lucky.

We frequently see advertisements that describe diamonds in fractional terms such as (one quarter carat, half carat, etc.) and wonder whether the actual weight of the diamonds fall within the legal guidelines.  Don’t be afraid to ask the sales clerk what the actual weight of the diamonds contained in the piece of jewelry is.  Your jeweler should be happy to provide you with the item’s actual diamond weight and will be impressed with the fact that you knew enough to ask.

Formulas for Calculating Diamond Carat Weight:

In the event that the carat weight of a diamond is unknown and an experienced clerk is not available to assist you, here are the GIA formulas and tables for calculating the carat weight of mounted diamonds:

Round Brilliant Cut Diamond Formula: the average diameter x the average diameter x the depth x 0.0061 = the approximate carat weight. To determine the average diameter of a diamond add the length plus the width and divide it by two. Keep in mind that these formulas are for approximation purposes only and that the actual weight of the diamond might be slightly different when unmounted.

If you’re staring at the formula and thinking “@#*! say what?!?!” it works like this.  Think of the surface of the diamond as if it were a clock and measure the distance from the twelve o’clock to the six o’clock position, let’s say it measures 6.48 mm.  Then measure the distance from the nine o’clock to the three o’clock position, let’s say that measures 6.51 mm.  Add 6.48 to 6.51 and you get 12.99 which you then divide by two (the number of the measurements taken) and the average diameter is 6.49 mm.  Measure the diamond from the table facet (flat facet on the top) down to the culet (bottom point), let’s say that is 3.96 mm and you’ll have the numbers required to run the formula.  For instance:

6.49 x 6.49 = 42.1850 x 3.96 = 167.052 x 0.0061 = 1.01 carats

The measurements used for this calculation were taken from an AGS Diamond Quality Document which indicates that the carat weight described on the lab report as measuring 6.48 – 6.51 x 3.96 mm was 1.014 carats.  Well I’ll be, would you look at that!  Go ahead.  Say it.

“Well I’ll be, would you look at that.”

Formula for Estimating the Carat Weight of an Oval Brilliant Cut Diamond:
average diameter x average diameter x depth x 0.0062 = approximate carat weight.

Formula for Estimating the Carat Weight of a Heart Shape Brilliant Diamond:
length x width x depth x 0.0059 = approximate carat weight.

Formula for Estimating the Carat Weight of a Triangular Brilliant (Trillion) Cut Diamond:
length x width x depth x 0.0057 = approximate carat weight.

Formula for Estimating the Carat Weight of an Emerald Cut, Princess Cut, and Quadrillion Cut Diamond:
length x width x depth x adjustment factor (see below) = approximate carat weight.

Adjustment Factor for Emerald, Princess, Radiant cut diamonds:  First determine the diamonds length to width ratio by dividing the diamonds length by its width and determining its relationship to 1.00  For example, if a diamonds length is 7.22 mm and the width of the diamond is 3.90 mm and it’s depth is 2.50 mm, then the length to width ratio would be calculated by dividing 7.22 by 3.90 which would equal 1.851282, which is then rounded to 1.85 and expressed as 1.85 to 1.00 or stated for appraisal purposes as 1.85:1.00 Once the length to width ratio is determined, use the adjustment factors below in the aforementioned formula to estimate the diamond’s approximate carat weight.

[table] [attr width=”20″],Length to Width Ratio,Adjustment Factor
1,1.25:1.00,0.0080
2,1.50:1.00,0.0092
3,2.00:1.00,0.0100
4,2.50:1.00,0.0106
[/table]

Marquise Cut Brilliant Diamond Formula: length x width x depth x adjustment factor.  In the example above, the diamond’s length to width ratio is 1.85:1.00 which does not appear on the table above.  Since the ratio is close to 2.00:1.00, you use 0.0110 as your adjustment factor.  To make your estimate more accurate, interpolate an adjustment factor between those for 1.50:1.00 and 2.00:1.00. Here, 0.0098 would be good.

[table] [attr width=”20″],Length to Width Ratio,Adjustment Factor
1,1.50:1.00,0.00565
2,2.00:1.00,0.00580
3,2.50:1.00,0.00585
4,3.00:1.00,0.00595
[/table]

Pear Shape Brilliant Cut Diamond Formula: length x width x depth x adjustment factor.

[table] [attr width=”20″],Length to Width Ratio,Adjustment Factor
1,1.25:1.00,0.00615
2,1.50:1.00,0.0060
3,1.66:1.00,0.00590
4,2.00:1.00,0.00575
[/table]

So there you have it, if you want to verify the carat weight of a mounted diamond all you have to do is ask the jeweler for a micrometer or the diamond’s dimensions and use the formulas above.  Note that approximate weights for common diamond sizes can be found within the user guide included with most micrometers.

Having a Little Fun Explaining Diamond Carat Weight:

Regardless of what you’ve been told by those who might want to spare your feelings, sooner or later it all boils down to size. No matter how beautiful the diamond is, some schmuck (like her dried up, old spinster of an Aunt) is going to Pin the Tail on the Donkey in front of the whole family and ask “So we’re all dying to know, uh, what’s his name? Uh Melvin, Shmelvin, uh whatever, Kevin, that’s it, Kevin! Anyway, so how BIG is it?”

Word of advice. She’s probably referring to the diamond.

Fatal, although possibly appropriate, responses to questions pertaining to the size of the diamond:

“It’s not the size, it’s how you mount it.”

“Bigger is not always better”

— Emphasized by staring at Auntie Spinster’s backside & shuddering.

“I went for quality, which apparently was not available back in your day.”

— Take a moment to squint at the Aunt’s never used, self purchased, promise ring and mumble something about needing to remember to get your eyes checked.

“Bigger than the hairy mole on your chin, but not as big as your…”

— Again, you have to physically shudder for this to work.

Ah yes, we digress, but no more or less than we would if we were sitting around discussing this subject in person… By now you’ve probably figured out that this is not your usual glossy version of the 4C’s outlined on those average jewelry store pamphlets, the truth is that we’re actually going to teach you something.

While we admit that the responses indicated above are probably accurate and possibly appropriate, they are not likely to make the transition into your new family easier. Therefore we advise strongly that you refrain from speaking your mind, regardless of the insight provided on this web site. If ignorance is bliss, then let the ignorant be, well, ignorant. The old bat really doesn’t deserve a response, but in an effort to maintain peace between opposing forces, we’ve crafted the following “more appropriate” excuses for not producing the Hope Diamond to Wow & Amaze the family.

“About a Bushel.”

Say this with total conviction because it is an absolutely viable explanation.   Diamond carat weight was originally measured using carob seeds as a comparison of weight with 100 carob seeds equaling one carat.  Each carob seed is referred to as a “point” so a diamond weighing 1.49 carats which is commonly referred to as “a carat and a half” is accurately represented as “149 points” which is equivalent to 149 carob seeds and that my friends sure sounds like “about a bushel” of carob seeds to me.

“I don’t know, but I liked that balance between it and the mounting.”

This is a rather easy explanation to defend because it is all a matter of personal preference and perception.  If the Spinster Aunt happens to say something along the lines of “well, the diamond doesn’t look large enough to me” you can say something about being able to upgrade to a larger diamond as you both get older and it becomes more difficult to see things without a magnifying glass.  Be sincere and polite, remember it’s never nice to be rude to your elders… you’re not implying that the Aunt can’t see, you’re just explaining the benefits of buying a diamond from a dealer with a great upgrade policy!

“It averages 6.5 mm in diameter.”

This is a kick because only a techno-geek will know that the average outside diameter of a 1.00 carat round shape diamond averages 6.5 mm. The family will hear 6.5 and spend the rest of the evening thinking that you were referring to the carat weight and second guessing the size of their diamond. If it’s a princess cut diamond, give them the tip-to-tip measurement as measured diagonally across the diamond because it sounds even more impressive.  Since diamonds come in all shapes and sizes, refer to the measurements section on the diamond grading report for the dimensions in millimeters.

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