Why the carat needed fixing in the first place
Before 1907, a "carat" was not one weight at all — it traced back to the carob seed, which traders once used as a rough natural counterweight because its seeds were assumed to be unusually consistent in mass. In practice they were not, and different trading centres had quietly settled on slightly different values for a carat: London, Amsterdam and various other markets each used a figure a few percent apart from the others. A stone weighed and invoiced in one city could come out a different number in another without anyone having made an error.
The fix was an international agreement, ratified at the 4th General Conference on Weights and Measures in 1907, that fixed the metric carat at exactly 0.2 grams everywhere. The United States adopted the same figure in 1913. Since then, "carat" has meant one thing worldwide, and every conversion on this page follows directly from that single defined number rather than from any measurement or estimate.
Points, milligrams, and how the trade actually talks
A carat divides into 100 points, so a stone described as "thirty points" is 0.30 carats and one described as "five pointer" is 0.05 carats — jewellers use points constantly for smaller stones because "twelve points" is easier to say and compare than "0.12 carats". Because a carat is exactly 200 milligrams, a point is exactly 2 milligrams, which is precise enough that laboratory scales built for gemmology are calibrated directly in these units rather than converted from grams on the fly.
The practical upshot: weight is the one property of a diamond or coloured stone this site is willing to state as a plain, unqualified fact, because it rests on a fixed unit definition rather than a graded opinion. Everything else that goes into a stone's value — colour, clarity, cut quality — depends on a report from a graded lab looking at the actual stone, which this page cannot see and does not attempt to estimate.
Carat and karat: two units, one confusing pair of spellings
Carat (ct) measures mass and is used for diamonds and coloured gemstones. Karat (K) measures purity — parts of pure gold out of 24 — and has nothing to do with weight. A description reading "2 carat, 18 karat" is using both words correctly for two completely unrelated properties of the same ring: the size of the stone, and the fineness of the metal holding it. Older British usage sometimes spelled the gemstone unit "karat" as well, which is the specific historical reason the two still get mixed up today, on top of the fact that they simply sound identical out loud.
A useful check: if a number sits near 9 to 24, it is almost certainly describing gold purity, since carat weights above about 5 are rare outside museum pieces and karat purity cannot exceed 24 by definition. If the number is written with a decimal point and applies to a stone rather than a metal, it is a carat weight.