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 more than
diamond. This means that a one-carat emerald will look larger
than a one-carat diamond, while the ruby will look smaller
than a diamond of the same weight. Emerald, with a mineral
composition that is lighter, will yield greater mass per
carat; ruby, with its heavier composition, will yield less
mass per carat.
Diamond prices are usually quoted per carat. Diamonds of
the finest and rarest quality are sold for the highest price
per carat, and diamonds of progressively less rare quality
are sold for a progressively lower price per carat.
Diameters and Corresponding Weight of Round and Brilliant