This is the easiest
"C" to understand. The weight of a diamond is directly related to
its size, and, although incorrect, these terms are sometimes used
interchangeably. The unit of weight for diamonds and other gemstones
is called the carat. This is a term which comes from the ancient use
of the very uniformlysized carob seed in measuring small weights.
A table of common weightrelated terms is given below.
Carat
Weight 
Equivalent
Weight 
One
Carat 
200 milligrams = 0.200 grams (There are 5
carats in 1 gram and 142 carats in 1 ounce.)
A carat is divided into 100 parts each called a point. So
1 carat = 100 points.

ThreeQuarter
Carat 
75
points 
Half
Carat 
50
points 
Quarter
Carat 
25
points 
One
Grain or "a one grainer" 
25
points or 1/4ct.; A diamond of approximately 1ct. would be called
a 4 grainer, one ~ 1.5ct. would be a 6 grainer, and a 0.5ct.
stone a 2 grainer. 
Melee 
Diamonds
weighing less than 1/5ct. (less than 20 points). Stones this
size are set using various techniques as groupings in jewelry. 
The relationship between how large a diamond looks and its actual
weight can be deceptive. For wellproportioned, round diamonds the
size (diameter)weight relationships are given below (The diamond
images are not actual sizes, and their proportional differences
are only approximate.). As examples, a 1.00ct. stone should normally
be about 6.5mm in diameter, a 3.00ct. would be 9.3mm, etc.
However, if a 1.00ct. round diamond has a shallow cut, sometimes
called a "spread cut", it will be will look larger than a wellcut
stone of the same weight because its diameter will be larger than
6.5mm. You might think that's great, but the trade off is that the
spread cut diamond will often have a lifeless, glassy look, called
a "fisheye".
Conversely, when a 1.00ct. round diamond is cut too deep, it appears
to be of smaller size than a wellcut stone, because the diameter
will be smaller than the expected 6.5mm. This means that you are
paying for extra weight in the pavilion and girdle areas, which
doesn't add to the beauty of the diamond. Furthermore, the stone
will often appear dark and dull in the center, resulting in what's
called a "nailhead".
The takehome lesson is that it is very important to consider
only a wellcut diamond, even, if necessary, at the expense of the
other 3C's (carat, color and clarity). Superior cut will give you
superior light performance producing a bright and lively stone.
It may not be the biggest looking diamond on the block, but it will
be the BEST looking diamond!
The same arguments concerning shallow and deep cut stones also
hold for the other fancy shaped diamonds, although the proportions
will be different. You can read about this subject in our article
on fancy shapes.
A final word about weight representation. In the U.S.,
the Federal Trade Commission requires that diamonds be weighed to
onethousandth of one carat (0.001ct.), but that the final weight
can be rounded to the nearest half point (0.005ct.). This means
that a diamond weighing 0.995ct. can be represented to you as a
1.00ct. stone. However, international convention is more strict,
and a weight can only be rounded to the next higher point (0.01ct.)
if the actual weight reaches 9/10ths (0.009ct.) of a point. For
example, the 0.995ct. diamond must be represented as a 0.99ct. stone,
and to be represented as a 1.00ct. diamond the stone must weight
0.999ct. before it can be rounded up to 1.00ct.
The GIA follows the more strict international convention in listing
the weights in their grading reports. AGS gives the actual weight
directly to 1/1000ct. in their grading reports. Dutton's Diamonds
follows and supports the more strict international convention in
carat weight representation.
Effect of carat weight on price: Because diamonds are more
rare the larger they are, the carat weight is not directly proportional
to the total asking price. For example, the total price of a 2ct.,
D, IF, stone is not twice the total price of an identical 1ct. stone,
but is more than three times the price, and an identically graded
3ct. diamond is about 7.7times as expensive as the 1ct. stone.
Also, a premium is added to diamond prices as they reach and exceed
each 1/4ct. increment in weight. (Hint: One way you might save a
little money is by buying stones with weights just below those increments,
if you can find them.)
