What Is Carat?
When talking about diamonds, one of the first questions is almost always the diamond’s weight, or carat (ct). Carat is one of the most important numbers on a diamond certificate because it affects both how the stone looks and how much it costs.
In this article, I’ll make carat simple: what carat really means, where the unit came from, how it affects a diamond’s appearance, why shape matters more than you think, and why price doesn’t increase linearly.
Quick Summary
1 carat (1.00 ct) = 0.2 g, or 200 mg.
Carat is weight, not diameter—though a higher weight usually also means a larger visual size.
Two diamonds with the same carat weight can look different because shape and cut change the visible surface area.
Price increases with carat not in a straight line, but in jumps (rarity).
A higher carat often means a taller setting and more attention to ring design.
What Is Carat?
Carat (ct) is the unit of weight used for diamonds and other gemstones.
1 ct = 200 mg = 0.2 g
Even though carat refers to the stone’s weight, it also affects visual size—because higher weight usually means larger measurements, especially with a round brilliant cut.
Where Did the Word “Carat” Come From?
The word “carat” comes from old trading practices. Long ago, gemstones were weighed using carob tree seeds (carob tree seeds) because their mass was considered consistently the same, around 2 g. Over time this practice became a standard, and eventually “carat” was standardized into the modern unit used worldwide today.
How Carat Affects a Diamond’s Visual Size
Carat tells you how heavy the stone is, but the “size” you see also depends on how the diamond is cut and what shape it is.
In general (for a round brilliant), the higher the carat, the larger the diameter—but:
if a diamond is cut too deep, it can be “heavier” but look smaller from the top
if a diamond is cut too shallow, it can look bigger but won’t sparkle as well
- if a diamond is cut to an ideal depth, it creates beautiful sparkle
How Does Diamond Shape Change the Perceived “Size”?
Two diamonds with the same carat weight can appear different in size depending on the shape, because the shape determines how much surface area the stone covers when viewed from the top.
Practical examples:
Oval: often looks larger and visually elongates the finger
Emerald / Radiant: longer and often lower proportions—can look larger and elongate the finger
Marquise / Pear: often look larger than their weight suggests because the shape stretches the outline
If your goal is visual size, choosing the right shape is one of the best ways to “play” with proportions.
Why Does Carat Increase Price So Quickly?
As carat increases, the price usually rises exponentially, not “proportionally.”
Why?
large diamonds are significantly rarer
even rarer is a large diamond with excellent cut, color, and clarity at the same time
certain “thresholds” (for example 1.00 ct) are psychological milestones in the market and push prices up
That’s why a 0.90 ct can sometimes be much better value than a 1.00 ct, even though the visible difference is small.
How Carat Affects Ring Design
The larger the stone, the more it affects the entire ring’s construction.
a larger diamond needs a stronger setting
the stone is often set higher (so the prongs can hold it securely)
proportions must be right: band width, stone height, overall balance
Large stones are visually very elegant and eye-catching—but precisely because of that, the design must ensure that:
it’s comfortable for everyday wear
the stone is protected
the ring doesn’t constantly snag
Summary
Carat is a simple number everyone asks about—but the best choice happens when you look at carat together with shape, cut, and proportions. The most beautiful result doesn’t always come from the “biggest” stone, but from the best-proportioned stone that fits your budget and style.