Weighing diamonds and a variety of other gemstones are done in metric carats: 1 carat is the equivalent of 0.2 grams, the weight is the same as a paperclip. (Don’t mix up carat with karat, as in “18K gold,” which is referring to gold purity.)
One carat is divided into 100 points just as a dollar is divided into 100 pennies. For example, the weight of a 50-point diamond is 0.50 carats. On the other hand, two diamonds with equal weight can have values that are very different based on the other members of the Four C’s: cut, color, and clarity. The weight of most diamonds used in fine jewelry is a carat or less.
Because even a fragment of a carat can make a huge difference in price, precision is crucial. Weight is often measured to the hundred thousandths of a carat and rounded to a hundredth of a carat in the diamond industry. The weight of diamonds greater than one carat is expressed in decimals and carats. (For example, a 1.08 ct. stone would be described as “one oh eight” or “one point oh eight carats,”)
HOW DID THE CARAT SYSTEM START?
The carat is the diamond and various other gemstones standard unit of weight and its name was taken from the carob seed. Since these tiny seeds had a fairly uniform weight, early gem traders used them in their balance scales as counterweights. The modern metric carat, equal to 0.2 grams, was adopted by the United States in 1913 and other countries soon after. Nowadays, the weight of one carat is exactly the same everywhere in the world.
The color of diamonds is about what you cannot see. The value of diamonds is determined based on how they closely come near colorlessness – they are higher value with less color. (The color pink and blue, which lie outside this color range is the exception to this fancy-color diamond).
The majority of diamonds in jewelry stores run to near-colorless from colorless, with small hints of brown or yellow.
It is the industry standard for GIA’s color-grading scale for diamonds The scale starts with the letter D, representing colorless, and continues with increasing presence of color to the letter Z, or near-colorless. Each letter grade has a clearly defined range of color appearance. Diamonds are color-graded by comparing them to stones of known color under controlled lighting and precise viewing conditions.
Most of these color differences are hard to see with the untrained eye. However, these small differences make a very big difference in diamond price and quality.
WHY DOES THE GIA COLOR GRADING SYSTEM START AT D?
Prior to GIA developed the D-Z Color Grading Scale, various other systems were applied loosely which included letters of the alphabet (A, B, and C, with multiple A’s for the best stones), Arabic (0, 1, 2, 3) and Roman (I, II, III) numerals, and descriptions such as “gem blue” or “blue white.” The outcome of all these grading systems was inaccuracy and inconsistency. Since the creators of the GIA Color Scale wanted a fresh start, with no connection with previous systems, they opted to begin with the letter D—a letter grade normally not linked with top quality.