When shopping for a polished diamond or diamond jewelry, like most people, you probably consider a diamond's size above all other factors. For example, when first contacting a jeweler, you may say that you are looking to purchase "a half-carat diamond," or maybe you want "a one-carat diamond."
According to the Gemological Institute of America (GIA), size is where many people start when contemplating a diamond purchase. However, over the years, partially due to the way diamonds are priced in the trade, a diamond's weight is what most of us have come to know and describe as a diamond's overall size. In actuality, diamond weight – specified by the metric carat – is not the same as a diamond's actual, overall size.
More accurately, according to the GIA, diamond "size" is the unique combination of both a diamond's weight and its dimensions. Moreover, by no means are two diamonds of the same carat weight – two one-carat, round brilliant diamonds, for example – exactly the same in their dimensions and proportions, even if they are both of the same shape.
All One-carat Diamonds are not Created – or Priced – Equally
For example, when comparing two one-carat (same weight), round (same shape), brilliant (same cut) diamonds, it is entirely possible for their measurements and corresponding proportions to be completely different. This discrepancy in dimension is how, when examining two diamonds of equal weight and similar shape, one diamond may look sensational while another not so much (due to its less-than-desired cut).
So, although we commonly express a diamond's size just by its carat weight, in truth, professionals in the diamond trade consider both a diamond's carat weight and its dimensions when determining its value relative to size (even though industry pricing is "weight driven").
It goes without saying that when judging stones, buyers should be aware not only of a diamond's weight but also its dimensions and proportions in order to fully appreciate the gems. These factors, along with a diamond's color grade; cut, finish and shape; clarity; crystal; and overall eye appeal and performance – including brilliance, scintillation and fire – affect a diamond's true value.
Magic Sizes Indicate Big Price Increases
According to GIA, the majority of diamonds in today's jewelry market weigh less than one carat. This is because larger-sized diamonds are very rare, and big diamonds become exponentially more scarce as their size increases. The fact that large diamonds are substantially more rare than the bulk of available gem-grade diamonds translates into higher costs for larger polished diamonds, both in the wholesale and retail markets. And, the jumps in cost can be significant.
In the trade, certain diamond weights are known as "magic sizes" and these weights correspond with substantial escalations, or jumps, in diamond pricing. The most well-known magic sizes include 0.50ct, 0.75ct, and 1.00ct. According to the GIA, this means, for example, that even when all other things are equal, there is about a 20-percent increase in value between a 0.96 ct diamond and a similar 1.02 ct diamond.
Diamonds weighing just slightly less than magic sizes are so desirable that the trade has nicknamed these just-shy stones; the trade term for a diamond weighing between 0.96 ct and 0.99 ct is a "light carat." A diamond weighing between 0.45 ct and 0.49 ct is called a "light half."
A savvy buyer looking for a diamond in the one-carat range, might seek out a well-cut diamond that weighs just under a carat, maybe even one with slightly larger "face-up" proportions (ie, a slightly shallower cut stone with larger girdle measurements that make it appear a little larger when viewed from above than its weight may indicate). The trick is to balance cost savings with a diamond's overall quality and eye appeal.
Cutters Aim for Beautiful, Weighty Diamonds
Also, be aware that most diamond cutters want to get the most weight (which translates into the most money) from their rough, especially in the magic size ranges. With that in mind, sometimes, a superbly cut "light carat" stone – one that didn't quite make the desired, pricier, magic, one-carat size – is a better gem than a full-carat stone that a cutter may have fashioned more for its higher-size value than for its beauty.
Of course, a "light carat" stone may be poorly cut as well. The point is, be mindful that in addition to trying to create beauty, cutters generally strive to achieve magic sizes; and sometimes, they make fashioning decisions that specifically affect a diamond's finished weight.
Total Weight and Total Gem Weight are not the Same
When you are considering a piece of diamond jewelry, understand that the terms "total weight" and "total gem weight" do not mean the same thing. According to the GIA, "total weight" refers to the combined weight of all the diamonds in a piece of jewelry (not just a center, featured, or primary diamond). By contrast, "total gem weight" means the combined weight of all stones in a piece of jewelry, not just diamonds.
Jewelry dealers may abbreviate or indicate these two weights differently, and it can be confusing. Examples of abbreviations – sometimes used indiscriminately – include: ctw, tcw, gtw, tgw, and tw. Be sure to understand what the indicated diamond and/or gem weight refers to, especially if you are making a purchase without seeing the jewelry in real life.
Diamond Dimensions and Proportions Indicate Cut Quality and Size
A diamond's unique measurements, or dimensions, make-up the other components of size. Diamond dimensions are precisely measured in thousandths, rounded to hundredths, of a millimeter (0.01mm). Combined, a diamond's measurements make-up unique proportions which indicate the overall quality and performance of a diamond's cut and shape – called its "make."
The most important diamond dimension is its depth, or the distance from the top of the stone (the table) to the bottom of the stone (the culet). Other key dimensions include the diameter for a round diamond, and the length and width for a fancy-cut diamond.
Regarding round brilliant diamonds, well-cut stones have fairly standard proportions. These measurements are helpful to know in terms of estimating weight and having somewhat of a benchmark when it comes to knowing what may be relatively "normal." For example, a 1.00 ct round brilliant often has a diameter of 6.50 mm.
Diamonds are Priced per Carat
Even though diamond size is determined both by weight and measurement, diamond wholesalers use a per-carat-price to quote the value of polished diamonds. The per-carat-price is the price of a gem divided by its weight. So, to determine a diamond's retail per-cart price, divide the total retail price by the diamond's carat weight.
Knowing how to roughly determine a loose diamond's per-carat-price is useful when comparing retail prices of different diamonds, especially when the diamonds may not be exactly the same weight. Recognizing differences in per-carat-prices may indicate better quality and/or value. Also, it may make it easier to determine where you need to be in terms of negotiating with your seller.
GIA Reports Identify Gem Weights and Measurements
GIA offers lab assessment and several different grading reports for individual diamonds and gemstones. Accurate carat weight, as well as gem dimensions are indicated on each certificate. In addition, some certificates provide details and illustrative descriptions regarding diamond and gemstone dimensions and proportions.
Claire Eddins - Writer/editor, award-winning photographer Claire Eddins collects gems, horses, and obsesses over home, art, and design.
