Diamond Carat Grading Diamonds are sold by the carat (ct), not
to be confused with karat (kt), which refers to gold purity.
Since 1913, most countries have agreed that a carat weights 200
milligrams, or 1/5 gram. Jewellers often refer to the carat
weight of diamonds in terms of points. This is particularly true
of stones under one carat. There are 100 points to a carat, so
if a jeweller says that a stone weighs 75 points, he means it
weighs 75/100 of a carat, or ¾ carat. A 25-point stone is ¼
carat. A 10-point stone is 1/10 carat. The carat is a unit of
weight, not size. We wish to stress this point, since most
people think that a one-carat stone is a particular size. Most
people, therefore, would expect a one-carat diamond and a one-
carat emerald, for example, to look the same size or to have the
same apparent dimensions. This is not the case. Comparing a one-
carat diamond with a one-carat emerald and a one-carat ruby
easily illustrates this point. First, emerald weighs less than
diamond, and ruby weighs mor