Diamonds are measured in terms of their weight, the terminology used to describe the weight of a diamond is a Carat. One carat is equal to 200 milligrams so a 2.00 carat diamond weighs 400 milligrams.
The carat weight of a diamond is the easiest of the characteristics of a diamond to determine since all that is required is a scale that is sensitive to milligram weights. However, the diamond must be loose in order to take that measurement accurately. Carat weight has no bearing whatsoever on the quality of the diamond or how much Brilliance, Fire and Scintillation a diamond will have.
The carat weight is related to the physical size of the diamond but this is a non linear relationship meaning that while the average size of a one carat Round Brilliant cut diamond may be 6.50mm, the proportioning of the diamond also plays a role in determining the physical size of that diamond.
For example, an Ideal cut one carat Round Brilliant diamond, which should have a Depth% between 59.0%-62.8% will measure on average 6.5mm. If not proportioned to Ideal diamond standards a deeply cut stone may measure as small as 6.00mm because the diamond is carrying a larger percentage of its carat weight on the underside of the diamond while a shallow cut stone may measure as large as 6.8mm or higher. So the actual physical size of any diamond is loosely related to the carat weight of the stone but it is the proportions that actually determine the proper size for a diamond of any given carat weight.
While the carat weight of a diamond will be a constant measure regardless of the shape of the diamond, the ideal proportioning of a diamond does depend on the shape of the diamond. Different shapes or cuts of diamonds have different Ideal sets of proportions and will also show weight differently than one another.
The Round Brilliant cut looks the largest relative to a given carat weight. A rectangular cut like the Radiant Brilliant or an oblong cut like the Oval or Marquise will look larger than a square Radiant or Princess cut.
Finally, Carat Weight is an important factor in determining value. The larger the stone, the more rare the stone and consequently the more valuable the diamond.
Additionally, there are price differences on the Rap sheet (the wholesale diamond pricing sheet) at major carat weight marks. A 1.00 carat diamond will not cost the same (per carat) as a 1.50 carat diamond with the same Color and Clarity characteristics.
Excellent values may be found at carat weights just under these break points.
A 0.99 carat diamond will cost less on a per carat basis than a 1.00 carat diamond with the same characteristics.