In the world of jewelry, every standard is based on near-absolute precision. That's why terms like... Carat (ct) and millimeter (mm) have become the common language among appraisers, craftsmen, and customers. However, despite their widespread use, these two concepts are often confused.

What is a Carat? Its origin and meaning in the world of jewelry.

Carat, the first element in the prestigious 4Cs (Carat, Cut, Color, Clarity) standardized by the Gemological Institute of America (GIA), is the foundation for valuing, classifying, and assessing the rarity of a gemstone. Understanding the carat value correctly is key to recognizing the true worth of a stone, and moreover, to appreciating the spirit of high-end jewelry artistry.

The term carat originates from the ancient Greek word " keration ," meaning "seed of the carob tree." Thousands of years ago, before the invention of precise scales, carob seeds (of nearly uniform size and weight) were used to weigh gemstones and gold. In the 19th century, as the diamond trade developed, an international standard was established: 1 carat = 0.2 grams (or 200 milligrams), a standard that has been maintained to this day.

Interestingly, although carat is a unit of weight measurement , in popular perception, most buyers tend to associate it with the size of the diamond. In reality, two diamonds weighing the same 1 carat can differ in size, depending on the cut. A deep-cut diamond, which retains more weight at the bottom, will appear smaller than a shallow-cut diamond, even if the weight remains the same.

How to measure and classify Carat

In the jewelry industry, the weight of a diamond is measured precisely to two decimal places (e.g., 1.02 carat). Even very small deviations, as little as 0.01 carat, can significantly affect the value of the stone, especially at benchmarks such as 0.50 carat, 1.00 carat, 1.50 carat, or 2.00 carat.

The reason lies in market psychology: a 0.99-carat diamond, although almost equivalent to a 1.00-carat diamond, is priced significantly lower. Therefore, artisans and dealers always value every thousandth of a gram, because it's not just about precision, but also about the art of assessing light.

Carat and the exponential pricing formula

The value of a diamond doesn't increase arithmetically but exponentially (exponentially) as its carat weight increases. This phenomenon is known as rarity compensation because large diamonds are extremely rare in nature. Finding a rough crystal large and of sufficient quality to craft a 1-carat diamond is many times more difficult than finding a 0.5-carat diamond.

For example, a 1-carat diamond (with the remaining 3Cs equivalent) is not worth twice as much as a 0.5-carat diamond , but can be three to four times more expensive . This dramatic price increase at these even weight points is an unwritten rule governing the global diamond market.

What is a "Ly"? The language of size in Vietnamese jewelry.

While carat is the global standard, "ly" is a familiar term in the Vietnamese jewelry industry. "Ly" refers to linear size , usually the diameter of the top facet of a diamond, measured in millimeters (1 ly = 1 mm = 0.1 cm).

For jewelers, the carat size is a visual indicator. It shows "how" the diamond will look when set in a ring, pendant, or earring. For customers, the carat size makes it easier to visualize the actual size of the stone than just looking at the carat number.

However, the "cup" (or "thickness") doesn't tell the whole story, nor does it fully reflect the value of a diamond. A 6.5 mm diameter diamond typically corresponds to 1 carat (if cut correctly), but that doesn't mean a 6.5 mm diamond always weighs exactly 1 carat. The cutting ratio strongly influences this data.

To help customers visualize, below is an approximate conversion table between weight (carat) and face diameter (mm) for Round Brilliant diamonds with an ideal cut :

For shapes other than round, elongated shapes like oval or marquise tend to have a wider weight distribution, creating the illusion that the stone looks larger than a round stone of the same weight.

Why can't Carat and Ly replace each other?

In gemological assessment, carat represents weight, and millimeters represent size , and these two factors are not linearly proportional. The reason lies in the geometric structure of the diamond.

Ideal Cut To maximize brilliance and fire (sparkle), the weight is perfectly distributed between depth and diameter. A standard 1-carat cut (e.g., round) will have a diameter of approximately 6.5mm.

Shallow cut diamonds have a shallow cut depth but a large diameter. A 0.90 carat shallow cut diamond can have a diameter of up to 6.8mm (larger than a standard 1-carat cut). However, light will not be totally reflected, making the diamond appear less brilliant and duller.

Deep Cut : A deep cut has a large depth but a small diameter. A 1.10 carat deep cut might only have a diameter of 6.2mm. The weight is hidden in the unseen areas, and light escapes from the edges, reducing the brilliance.

Carat refers to the inherent value and rarity of the stone, while ly indicates its visual size and how it will be set in jewelry.