The 4 C's of Diamonds: Carat (Weight)
The 4 C's Diamond Grading System
Carat weight is one of the 4 C's, representing the four variables that are used to calculate the quality and value of a diamond. Both rough and cut diamonds are separated and graded based on these four characteristics. As a consumer, your first step in shopping for a diamond should be to learn and understand the "4 C's" diamond grading system.
A diamond or gemstone's "Carat" designation is a measurement of both the size and weight of the stone. One "Carat" is a unit of mass that is equal to 0.2 grams (200 milligrams or 3.086 grains) or 0.007 ounce. A carat can also be divided into "points" with one carat being equal to 100 points, and with each point being 2 milligrams in weight. Therefor, a 1/2 carat diamond would be 50 points, a 3/4 carat diamond is 75 points, and a 2 carat diamond is 200 points.
When a single piece of jewelry has multiple stones, the total mass of all diamonds or gemstones is referred to as "Total Carat Weight" or "T.C.W."
The word "Carat" is derived from the Greek word keration, or "seed of the carob". In ancient times, carob seeds were used to counterbalance scales, and as a benchmark weight due to their predictably uniform weight.
Note: Your screen resolution may alter the reproduction size of the chart above. This carat/millimeter sizing chart is meant for comparison purposes only.
Occasionally, a stone cutter will need to make compromises by accepting imperfect proportions and/or symmetry in order to avoid noticeable inclusions, or to preserve the carat rating of the rough stone. Since the per-carat price of diamond is much higher when the stone is over one carat, many one carat diamonds are the result of compromising cut quality to increase carat weight. It is for this reason that an even 1.00 carat diamond may be a poorly cut stone.
Some jewelry experts advise consumers to purchase a .99 carat diamond for its better price, or to buy a 1.10 carat diamond for its better cut. See the chart above for a millimeter to carat size comparison.
Think of the "spread" as the apparent size of a diamond. By sacrificing cut proportions and symmetry, a diamond can have a larger diameter and therefor, a larger apparent "size" for a given carat weight. The spread is the ratio between diameter and three principle geometric components of the crown, girdle and pavilion. A given diamond will have a 'zero spread penalty' if the correct 'ideal cut' symmetry of a 32.5¼ crown, 40¼ pavilion, 58% table and 1% girdle are maintained.
According to the Federal Trade Commission's (FTC's) Jewelry Guides on Decimal Representations, "If the diamond's weight is described in decimal parts of a carat, the figure should be accurate to the last decimal place." If the carat weight is shown as ".20 carat" could represent a diamond that weighs between .195 and .204 carat.
If the carat weight is shown as one decimal place, it must be accurate to the second decimal place. A diamond that has a specified carat weight of .5 carats must have an actual weight of between .495 carats and .504 carats.
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Diamond prices do not increase in a steady line, as each jump past a even carat weight can mean a significant jump in pricing. The "Rapaport Diamond Report" is a weekly diamond price list based on cut, clarity and weight, that is published by the Rapaport Group of New York.
Independent Diamond Testing Laboratories
Diamond Chemistry & Composition
Rapaport.com - Rapaport Diamond Report