Diamonds are graded in four categories know as the four C’s (carat, colour, clarity, and cut).
About the four C’s
Diamonds and other gemstones are weighed in metric carats: one carat is equal to 0.2 grams, about the same weight as a paperclip. (Don’t confuse carat with karat, as in “18K gold,” which refers to gold purity.) Just as a pound is divided into 100 pennies, a carat is divided into 100 points. For example, a 50-point diamond weighs 0.50 carats. But two diamonds of equal weight can have very different values depending on the other members of the Four C’s: clarity, color and cut. The majority of diamonds used in fine jewelry weigh one carat or less.
Because even a fraction of a carat can make a considerable difference in cost, precision is crucial. 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 1.08 ct. stone would be described as “one point oh eight carats,” or “one oh eight.”)
Diamond color is all about what you can’t see. Diamonds are valued by how closely they approach colorlessness – the less color, the higher their value. (The exception to this is fancy-color diamonds, such as pinks and blues, which lie outside this color range.)
Most diamonds found in jewelry stores run from colorless to near-colorless, with slight hints of yellow or brown.
GIA’s color-grading scale for diamonds is the industry standard. 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.
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.
Because diamonds formed deep within the earth, under extreme heat and pressure, they often contain unique birthmarks, either internal (inclusions) or external (blemishes).
Diamond clarity refers to the absence of these inclusions and blemishes. Diamonds without these birthmarks are rare, and rarity affects a diamond’s value. Using the GIA International Diamond Grading System™, diamonds are assigned a clarity grade that ranges from flawless (FL) to diamonds with obvious inclusions (I3).
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.
Cut is the factor that fuels a diamond’s fire, sparkle and brilliance.
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. The allure of a particular diamond depends more on cut than anything else.
Though extremely difficult to analyze or quantify, the cut of any diamond has three attributes: brilliance (the total light reflected from a diamond), fire (the dispersion of light into the colors of the spectrum), 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.