December 21, 2020
Simply put, a diamond's carat weight refers to how much it weighs. One metric carat is defined as exactly 200 milligrams. So a 1-carat diamond weighs 200 milligrams.
Each carat is further divided into 100 points. This allows for very precise measurements to the hundredth decimal place. For example, a diamond weighing 1.08 carats may be described as "one point zero eight carats" by a jeweler.
The relationship between carat weight and diamond size is not perfectly linear. As the carat weight increases, the diamond's dimensions increase at a slower rate. This is because larger diamonds are cut to retain as much weight as possible in the finished stone. The cut proportions are optimized for carat weight over spread.
The diamond carat system originated from the carob seed. In ancient times, traders used the small, uniform seeds as counterweights on precision balances for weighing gems. The modern metric carat weight corresponds almost exactly to the average mass of a carob seed.
Over time, the carat was standardized based on metric weight to be exactly 200 milligrams worldwide. The legacy of the carob seed remains with us today in the terminology of the diamond industry.
In diamond weight, certain increments are considered more desirable or "magic." These include half-carat (0.50 ct), three-quarter carat (0.75 ct), and the full one-carat sizes.
Visually, there is little apparent difference between a 0.99 ct diamond and a 1.00 ct diamond. However, that tiny difference of 0.01 ct can have a significant impact on the diamond's market value and price. Retail customers tend to place greater value on crossing these "magic size" thresholds.
Larger diamonds are exponentially more rare in nature than smaller ones. It requires a much larger rough diamond crystal to cut a sizeable finished stone. This scarcity is why carat weight is tied so directly to the price of diamonds.
Bigger diamonds also face more risk of inclusions and flaws in the rough crystal that can affect clarity. It takes an extraordinarily rare find to yield a large diamond with top color and clarity grades. Thus, the combination of size along with quality factors results in a tremendous premium for large flawless diamonds.
As with the other 3Cs - cut, clarity, and color - carat weight is an important factor to consider when selecting a diamond. But bigger is not always better if it comes at the expense of cut and clarity. The right balance depends on your budget and preferences. Discuss your needs with a jeweler to ensure you get the ideal diamond carat weight for your engagement ring or other jewelry.
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