A carat is a unit of weight, not size. One carat equals 0.2 grams, about the weight of a paper clip, and every gemstone in the world is weighed on that same scale. What most shoppers really want to know is something slightly different: how big will it look, and is it worth the price?

Carat Is Weight, and Size Follows

The word comes from the carob seed, which traders once used as a counterweight on gem scales. Today a carat is fixed at exactly 200 milligrams and split into 100 points. A jeweler might call a 0.25 carat diamond "a quarter carat" or "a 25-pointer"; both mean the same thing.

Because carat is weight, size depends on the stone's shape and how it was cut. A well-cut round diamond spreads its weight across the face you see. A deep, heavy cut hides weight underneath, so it can weigh the same yet look smaller from above.

Carat Weight to Millimetres: Round Diamonds

These are typical face-up diameters for well-cut round brilliant diamonds. Small differences are normal from stone to stone.

Notice that doubling the weight from 1 to 2 carats does not double the diameter. It adds about 1.7 mm, because the extra weight also goes into depth.

What "ct. tw." Means on Earrings and Bands

"ct. tw." stands for carat total weight: the weight of every stone in the piece added together. A pair of 1 ct. tw. diamond studs holds two diamonds of about 1/2 carat each. A band with 20 small diamonds marked 1/2 ct. tw. has stones of roughly 2.5 points each. Always check whether a weight is per stone or total before comparing prices.

Lab-Grown Diamond Stud Earrings in 14K White Gold (1/2 ct. tw.)$347.99View pieceLab-Grown Diamond Stud Earrings in 14K White Gold (3/4 ct. tw.)$443.99View pieceLab-Grown Diamond Stud Earrings in 14K White Gold (1 ct. tw.)$541.99View piece

Why Price Jumps at Certain Weights

Larger rough crystals are rarer, so price per carat rises with size. It also steps up at popular round numbers, the so-called magic sizes: 0.50, 0.75, 1.00 and 2.00 carats. A 0.95 carat diamond can cost noticeably less than a 1.00 carat stone while looking virtually identical, since the size difference is a fraction of a millimetre.

Lab-grown diamonds follow the same weight scale but cost far less per carat, which is why many shoppers now choose a larger lab-grown stone for the budget they would have spent on a smaller mined one. Both are real diamond, with the same hardness and sparkle.

Carat Is Only One of the Four Cs

Carat tells you how much diamond you have. The other three Cs decide how good it looks:

• Cut controls sparkle. A well-cut smaller stone often outshines a poorly cut larger one.
• Color runs from D (colorless) down the alphabet. G to H looks colorless to most eyes once set.
• Clarity grades tiny internal features. VS and SI stones look clean without magnification.

A sensible balance for most budgets: an excellent cut first, then the largest carat weight that fits, with color and clarity in the "looks clean and white" range.

Lab Grown Diamond Solitaire Pendant, 1/4 Carat Total Weight, Round Cut, VS/SI Clarity, DEF Color, 14K Gold$154.99View piece

Colored Gemstones Weigh Differently

Carat is the same unit for every gem, but gems have different densities. Sapphire and ruby are denser than diamond, so a 1 carat sapphire looks a little smaller than a 1 carat diamond. Amethyst and other quartz are lighter, so a 1 carat amethyst looks larger. That is why colored stones are often described by millimetre size, such as a 6 x 4 mm oval, which tells you more about how they will look.

How to Choose the Right Carat Weight

1. Start with how it will be worn. Everyday studs and pendants look polished at 1/4 to 1/2 carat. Statement pieces start around 3/4 carat.
2. Think in millimetres. Use the table above, and compare with something familiar: a pencil eraser is about 6.5 mm, close to a 1 carat round.
3. Shop just under the magic sizes for more value with no visible difference.
4. Consider lab-grown if a larger look matters more to you than a mined origin.

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