Although many people think of a carat (ct) as a measure of diamond or gemstone size, a carat is not a particular size. It is, in fact, a specific measure of metric weight. Also, the carat is different from the karat (kt) which is a United States measurement for gold purity.
Carat measurement is paramount to the diamond and gem industry because in just about every level of transaction and trade – often with the exception of final retail consumer sales – diamonds and gems are sold on the basis of their weight per carat. Savvy buyers know current values of particular stones and understand how to negotiate from there, whether shopping for loose stones, new jewelry, antique or pre-owned jewelry.
History of Carat Measure in Gem Trading
Since ancient times, the standard weight used for jewels in the gem trade has been the "carat." In "The New International Diamond Carat of 200 Milligrams," Transactions of the American Institute of Mining Engineers, vol. 47, 1914, gem expert George Frederick Kunz writes that before the year 1913, the carat varied in weight from country to country. Early gem traders around the world used tiny, relatively uniform grains or seeds to measure small and precious items such as gemstones. Many traders planted grains and seeds specifically to harvest and use as measurements of weight. Carob and wheat seeds were particularly notable as units of measure for weight.
According to the Gemological Institute of America, today's modern pearl grain, troy grain, and avoirdupois and apothecaries' grains all descended from the wheat grain. Moreover, the modern metric carat evolved from the ancient carob seed.
Although the term "carat" was relatively universal, the exact weight of a carat was not. Around the world, these ancient, grains and seeds varied in size and weight making carat measurement unequal from place to place. For example, according to Encyclopedia Britannica, at one time the London carat was equal to 3.168 troy grains, or 0.2053 gram, while the Florence carat was 0.1972 gram, and the Amsterdam carat was 0.2057 gram.
Also, in earlier times, the weight of a carat was calculated by whole carats plus fractions (1/2, 1/4, 1/8. 1/32. or 1/64) of a carat. In general terms, this practice continues today. However, actual weights of loose diamonds and gemstones are calculated quite precisely.
Standardized Carat Weight for Gems
In 1913, led by the French Conférence Générale des Poids et Measures, an international assembly concerned with weights and measures, many countries around the world came together and agreed to define the carat as a metric unit of weight that is equal to 1/5 gram, or 200 milligrams. This is the measurement used in the gem and jewelry industry today, according to the Bureau International des Poids et Measures, the governing body responsible for the international conference.
Because fractions of a carat may represent hundreds or even thousands of dollars, diamonds are measured by thousandths of a carat and rounded to the nearest hundredth, or point. There are 100 points to a carat. So, for example, a half-carat stone is 50 points. In the United States, The Federal Trade Commission strictly regulates how the jewelry industry represents gemstone weight.
When purchasing jewelry featuring more than one diamond or gemstone, be aware of the difference between "ct" which means carat (singular), "cts" (plural) and "ctw" which stands for "carat total weight" and refers to the total weight of all stones combined, not just a central or primary stone in a piece.
Old Diamonds May Weigh "More"
One important note for buyers of antique gems: in early days, a carat could actually weigh more than the modern metric carat. Be on the lookout for this if you are shopping for antique jewelry or gems, especially those offered for sale that may include original reports or certificates of sale. For example, an old European cut diamond in an antique ring offered with the original receipt (specifying the diamond as 1.50 cts), most likely will be larger than 1.50 cts by today's standards.
Also, the only way to accurately ascribe a weight to a diamond or gemstone is to weigh the stone loose, using proper equipment, before it is set into jewelry. Hence, unless a jeweler has unset and reset a gemstone in a jewelry piece (especially a pre-owned or an estate piece), or there is a certificate that accompanied the loose stone before it was set into jewelry, the specified weight of a gemstone set into jewelry should be considered an estimate.
How Carat Weight Impacts Diamond and Gem Pricing
As stated earlier, carat weight is important in the diamond and gem industry because gemstones are priced and sold by their per-carat weight. Simply put, the finer and scarcer the gemstone, the higher the price-per-carat. To that end, as a gem increases in size, the price-per-carat usually increases because in most cases, bigger, gem-quality stones are more rare than smaller stones.
Also, in the majority of gem varieties, the escalation in per-carat price does not increase in direct relation to the increase in size. Usually, price increases are much greater than the corresponding increases in gem size. For example, although two-times the size in weight, the 2-ct diamond will be well-more that twice the price of the 1-ct diamond. In fact, a larger gem can be three, ten or many more times as expensive as a smaller stone of similar quality. A 3-ct diamond will be even more disproportionate in price to both the 1-and 2-ct stones.
Diamond Size vs. Weight: Face-up and Spread May Mask True Weight
These days, there is an impression of standard-sizing for diamonds, meaning many people think that all 1-ct diamonds have the same measurements. Not true. As mentioned earlier, there is no particular size that correlates to a gemstone's particular weight.
When referring to diamond and gemstone "size," one is usually referring to the size of the girdle outline. The girdle outline size is sometimes called the "spread" or "face-up size."
The misconception that all 1-ct diamonds are the same specific size, for example, is partially born out of the diamond industry's aggressive marketing, and consumer preference for round brilliant cut diamonds of a particular style (mostly in diamond engagement rings). So, although it may be safe to use, let's say, 6.5mm, as an average benchmark measurement for an average -ct round brilliant diamond, this is only an estimate in terms of size. A 1-ct diamond that is cut shallower, will "face-up" larger, meaning the diameter of the girdle will be larger, and, perhaps, when viewed from above, the diamond will appear to be as much as 1.25 cts. A more deeply cut 1-ct stone will show a smaller "spread" and may not look as big as its official 1-ct weight.
Consider Face-up Size in Addition to Weight
Compared to diamonds shaped and faceted in the round brilliant styles, sizing for fancy diamond cuts – including pear, marquise, radiant, princess, emerald and step cut, as well as others – is more difficult to generalize in terms of expected size. For this reason, it is often more helpful to shop for a particular face-up size in millimeters, than it is to shop for a specific carat weight. Also, certain faceting patterns, like the square princess cut, for example, are known to be a bit deeper, hence their face-up size will generally be smaller than a similarly weighted round brilliant cut.
Knowing that many beautifully cut smaller diamonds face-up larger than their expected size, can lead to a happy price advantage for a smart buyer.
Why Sapphire is 'Smaller' than Diamond
One final note, because different gemstones have unique mineral compositions, their densities, or specific gravities, are not the same. Hence, differing types of gemstones usually have different carat weights when their face-up millimeter sizes may be similar.
For example, the specific gravity for diamond is 3.5 and the specific gravity for corundum, or sapphire, is 3.9-4.1. The sapphire is heavier than the diamond, so a 1-ct sapphire will be smaller in size than the same-weight 1-ct diamond. By contrast, the specific gravity for emerald is 2.6, making a 1-ct emerald the largest gem of the three 1-ct gemstones.
Claire Eddins - Writer/editor, award-winning photographer Claire Eddins collects gems, horses, and obsesses over home, art, and design.
