Diamonds and other gemstones are weighed in metric carats:
One carat is equal to 0.2 grams. Just as a dollar is divided into 100 pennies, a carat is divided into 100 points. For example, a 18-point diamond weighs 0.18 carats. But two diamonds of equal weight can have very different values depending on the other factors of the Four C’s: clarity, color and cut.
In the diamond industry, weight is often measured to the hundred thousandths of a carat, and rounded to a hundredth of a carat. Diamond weights greater than one carat are expressed in carats and decimals. (For instance, a 1.18 ct. stone would be described as "one point one eight carats,” or "one one eight.”)
HOW DID THE CARAT SYSTEM START?
The carat, the standard unit of weight for diamonds and other gemstones, takes its name from the carob seed. Because these small seeds had a fairly uniform weight, early gem traders used them as counterweights in their balance scales. The modern metric carat is equal to 0.2 grams. Today, a carat weigh is exactly the same in every markets of the world.
COLOR
Most diamonds found in jewelry stores run from colorless to near-colorless, with slight hints of yellow or brown.
There are many color-grading scale systems for diamonds. The most commonly use is GIA system, the scale begins 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. Some systems use 0 to 10, the 0 for colorless and the same idea as GIA, continues with increasing presence of color to 10. Some may use "Exceptional white until Tinted.” All systems are the same idea, colorless is the best color for diamond grading.
Many of these color distinctions are so subtle as to be invisible to the untrained eye. But these slight differences make a very big difference in diamond quality and price.
WHY DOES THE GIA COLOR GRADING SYSTEM START AT D?
Before GIA developed the D-Z Color Grading Scale, a variety of other systems were loosely applied. These 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 result of all these grading systems was inconsistency and inaccuracy. Because the creators of the GIA Color Scale wanted to start fresh, without any association with earlier systems, they chose to start with the letter D - a letter grade normally not associated with top quality.
CLARITY
Diamond clarity refers to the absence of these inclusions and blemishes.
Diamonds without clarity are rare, and rarity affects a diamond’s value. Every diamond is unique. None is absolutely perfect under 10× magnification, though some come close. Known as Flawless diamonds, these are exceptionally rare. Most jewelers have never even seen one.
The GIA Diamond Grading System, diamonds are assigned a clarity grade that ranges from flawless (FL) to diamonds with obvious inclusions (I3).
The GIA Clarity Scale contains 11 grades, with most diamonds falling into the VS (very slightly included) or SI (slightly included) categories. In determining a clarity grade, the system considers the size, nature, position, color or relief, and quantity of clarity characteristics visible under 10× magnification.
Flawless (FL) - No inclusions or blemishes are visible to a skilled grader using 10× magnification
Internally Flawless (IF) - No inclusions and only blemishes are visible to a skilled grader using 10× magnification
Very, Very Slightly Included (VVS1 and VVS2) - Inclusions are difficult for a skilled grader to see under 10× magnification
Very Slightly Included (VS1 and VS2) - Inclusions are clearly visible under 10× magnification but can be characterized as minor
Slightly Included (SI1 and SI2) - Inclusions are noticeable to a skilled grader using 10× magnification
Included (I1, I2, and I3) - Inclusions are obvious under 10× magnification and may affect transparency and brilliance
CUT
Cut is the factor that causes a diamond’s brilliance, fire and scintillation.
The traditional 58 facets in a round brilliant diamond, each precisely cut and defined, are as small as two millimeters in diameter. But without this precision, a diamond wouldn’t be nearly as beautiful.
Though extremely difficult to analyze or quantify, the cut of any diamond has three attributes: brilliance (the light reflected from a diamond), fire (the dispersion of light into the spectral colors), and scintillation (the flashes of light, or sparkle, when a diamond is moved).
An understanding of diamond cut begins with the shape of a diamond. The standard round brilliant is the shape used in most diamond jewelry. All others are known as fancy shapes. Traditional fancy shapes include the marquise, pear, oval and emerald cuts. Hearts, cushions, triangles and a variety of others are also gaining popularity in diamond jewelry.
As a value factor, though, cut refers to a diamond’s proportions, symmetry and polish. For example, look at a side view of the standard round brilliant. The major components, from top to bottom, are the crown, girdle and pavilion. A round brilliant cut diamond has 57 or 58 facets, the 58th being a tiny flat facet at the bottom of the pavilion that’s known as the culet. The large, flat facet on the top is the table. The proportions of a diamond refer to the relationships between table size, crown angle and pavilion depth. A wide range of proportion combinations are possible, and these ultimately affect the stone’s interaction with light.
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