Carat measures a diamond's weight, not its size. One carat equals 200 milligrams or one-fifth of a gram. The term comes from carob seeds. Ancient gem traders used these uniform seeds as counterweights on their scales.
The metric carat became the universal standard in 1907. Today, every diamond is weighed on precision scales accurate to the hundred-thousandth of a carat.
Each carat divides into 100 points. A jeweller might call a 0.75-carat diamond a "seventy-five pointer." Diamonds over one carat are described in carats and decimals, like 1.50 carats or 2.08 carats.
Carat Measures Weight, Not Size
This is the most common confusion. Two diamonds can weigh exactly one carat but look completely different in size.
Different gemstones have different densities. A one-carat ruby looks smaller than a one-carat diamond because ruby is denser. It packs more weight into less space.
Even among diamonds, weight doesn't guarantee size. A poorly cut diamond can hide excess weight below the girdle, the edge where the top and bottom meet. This hidden weight costs you money but adds nothing to how the diamond looks when worn. A badly cut 1.20-carat diamond might appear identical to a well-cut 1.00-carat stone.
Shape affects appearance too. Elongated cuts like oval, pear, and marquise spread their weight across a larger surface area. They look bigger than round diamonds of the same weight. A one-carat oval can appear as large as a 1.25-carat round.
How Carat Weight Affects Price
Larger diamonds are rarer. Price increases exponentially with weight, not linearly. A one-carat diamond costs more than twice what a half-carat diamond costs, assuming the same quality in cut, color, and clarity.
The industry recognizes "magic sizes" at certain weight thresholds—0.50, 0.75, 1.00, 1.50, and 2.00 carats. Prices jump dramatically at these marks because demand concentrates here.
A 0.97-carat diamond and a 1.03-carat diamond look virtually identical. You couldn't tell them apart without weighing them. But the 1.03-carat stone costs significantly more because it crosses the one-carat threshold.
Smart buyers shop just below magic sizes. A 0.90-carat diamond can cost 10-20% less than a 1.00-carat stone of identical quality. They appear almost the same size. The savings can be substantial.
Carat Weight vs. Total Carat Weight
When shopping for jewellery with multiple diamonds, you'll see abbreviations like CTW, TW, or CTTW. These stand for Carat Total Weight or Total Carat Weight.
CTW refers to the combined weight of all diamonds in a piece. A ring described as "1.00 CTW" might have a 0.70-carat center stone plus 0.30 carats in smaller side stones.
This matters because total weight can mislead. A piece advertised as "2 carats total weight" sounds impressive. But if that's 40 tiny stones of 0.05 carats each, the visual impact differs greatly from a single two-carat diamond.
Always ask about the center stone's individual weight when evaluating jewelry with multiple diamonds.
Where Carat Weight Fits in Value
Carat weight is one of the 4Cs, the others being cut, clarity, and color. But bigger isn't always better.
Cut quality determines how a diamond handles light. A poorly cut two-carat diamond can look dull and lifeless. A beautifully cut one-carat stone can outshine it completely. The sparkle comes from cut, not weight.
Most engagement rings fall between 0.50 and 1.50 carats. The average sits around 0.80 carats. Anything above two carats is considered large and commands premium pricing due to rarity.
When balancing the 4Cs within a budget, many buyers prioritize cut quality first. They then choose a carat weight just below a magic size threshold. Finally, they select the lowest color and clarity grades that still look excellent to the naked eye.
This approach maximizes visual impact while managing cost. You get a diamond that looks beautiful in real life without paying for differences only visible under magnification or on a certificate.
Don't Confuse Carat with Karat
Carat measures diamond weight. Karat measures gold purity. The spelling differs by one letter but the meanings are completely unrelated.
Pure gold is 24 karat (24K). An 18-karat gold ring contains 18 parts gold and 6 parts other metals, making it 75% pure gold.
This confusion is common but important to clear up when discussing jewellery .
Thanks for reading
Jared & Brie