Weight representation
In the U.S., the Federal Trade Commission requires that diamonds be weighed to one-thousandth 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.
Value per carat
increases with carat size, because larger rough diamonds occur less frequently. In other words, 2 half-carat diamonds taken together will not cost as much as 1 one-carat diamond, as the one-carat stone is more rare. Also, a premium is added to diamond prices as they reach and exceed each 1/4ct. increment in weight.
Color
The color grade of a diamond refers to how closely its body color approaches colorlessness. The best color for a colorless diamond is, in fact, an absence of color. A truly colorless stone will carry a premium price, and the larger that diamond, the greater the premium per carat.
Most diamonds have at least a trace of yellow, brown, or gray body color. Though a lot of diamonds will appear to be colorless, most actually possess subtle shade differences. These variances in color are due to traces of elements such as nitrogen and boron that become assimilated into a diamond's atomic structure during the original formation. Most commonly, a diamond will have a hint of yellow or brown due to traces of nitrogen.
