De fleste mennesker tror, at karat refererer til den faktiske størrelse af en diamant, når det faktisk er en standardmålenhed, der definerer vægten af en diamant. En karat svarer til 200 milligram. Karatstørrelser udtrykkes også som "punkter", med en en karat diamant svarende til 100 punkter, a one-half carat diamond being 50 punkter, a three-quarter carat diamond being 75 punkter, og så videre. Larger diamonds are much rarer than smaller ones.
Since a carat is a unit of measure and not size, two diamonds of the same carat weight may appear to be different sizes depending on how the diamond is cut. Some diamonds will have extra weight on the bottom part — or pavilion — of the stone, and therefore appear smaller. A premium cut Canadian diamond is perfectly cut, and will appear larger than many diamonds of a heavier carat weight.
If size or carat weight is your most important attribute, (for many people it is very important) that is perfectly acceptable. You may want to also consider the other 3 Cs to make sure you understand your tradeoffs.