The Basics: CARAT, COLOR, CLARITY, CUT.
Developed in the 1940s and 50s by the Gemological Institute of America (GIA) the four C’s are a universal, four-part grading system for diamond quality. A diamond’s value is based on how these factors interplay with each other, meaning it’s not just about the size of the diamond. Bigger, in many cases, is not always better.

CARAT:
Diamonds and other gemstones are weighed in metric carats: one carat is equal to 0.2 grams, about the same weight as a paperclip. (Note: carat should not be confused with karat, as in “18k gold”, which measures gold purity.) Carats refer to the number of points on a diamond. Just like there are 100 pennies in a dollar, there are 100 points on a one-carat diamond. So a 50-point diamond weighs 0.50 carats and a 75-point diamond weighs .75 carats. Because diamonds are so valuable, carats are measured to the hundred thousandth of a carat, then rounded to the nearest hundredth. Diamonds larger than one-carat are noted in decimals. A 1.08 diamond would be pronounced: “one point oh eight” or just “one oh eight”.

The term carat is derived from carob seeds which early gem traders used as counterweights to their balance scales due to the seeds’ generally uniform weight. The modern metric carat, equal to about 0.2 grams, was adopted in the US in 1913 and other countries soon after. Today, carat weight is universal so a diamond’s carat weight is the same in every country.

COLOR:
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 – with the exception of 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 and brown.

GIA’s color-grading scale for diamonds is the industry standard. The scale begins with the letter D (earlier grading systems already used A, B and C), representing ‘colorless’ diamonds, and continues with increasing presence of color to the letter Z, or ‘near-colorless.’

Each letter grade has a specific and 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 diamond color distinctions are so subtle that they’re invisible to the untrained eye, but even the slightest color variations make a big difference in diamond quality and diamond price.

CLARITY:
Because diamonds are formed deep within the earth under extreme heat and pressure, they often contain unique birthmarks – either internal marks knows as “inclusions” or external marks known as “blemishes”. Diamond clarity refers to the absence of inclusions and blemishes. Diamonds without any markings whatsoever are extremely rare and rarity increases a diamond’s value. The GIA International Diamond Grading System™ assigns diamonds a clarity grade that ranges from flawless (FL) to diamonds with obvious inclusions (I3).

The GIA Clarity Scale has 11 grades, with most diamonds falling into the VS (very slightly included) or SI (slightly included) categories. In determining a clarity grade, the GIA system considers the size, nature, position, color or relief, and quantity of clarity characteristics visible under 10x magnification. Flawless diamonds are exceptionally rare and most jewelers have never seen one (though we have one!)

Here’s the complete scale:

• Flawless (FL) – No inclusions or blemishes are visible to a skilled grader using 10x magnification.
• Internally Flawless (IF) – No inclusions and only blemishes are visible to a skilled grader using 10x magnification.
• Very, Very Slightly Included (VVS1 and VVS2) – Inclusions are difficult for a skilled grader to see under 10x magnification.
• Very Slightly Included (VS1 and VS2) – Inclusions are clearly visible under 10x magnification but can be characterized as minor.
• Slightly Included (SI1 and SI2) – Inclusions are noticeable to a skilled grader using 10x magnification.
• Included (I1, I2 and I3) – Inclusions are obvious under 10x magnification and may affect transparency and brilliance.

CUT:
The allure of a particular diamond depends more on cut than anything else because cut is the factor that fuels a diamond’s brilliance, sparkle and fire.

The traditional 58 facets in a classic brilliant round diamond—precisely cut and defined—are as small as 2mm in diameter. 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 total light reflected from a diamond), Fire (the dispersion of light into the colors of the spectrum) and Scintillations (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 most often in diamond jewelry. All others are known as “fancy shapes,” including: 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 is possible, and these ultimately affect the diamond’s interaction with light. A pavilion depth that’s too shallow or too deep will allow light to escape through the sides of the bottom of the stone, minimizing brilliance, sparkle and fire. A well-cut diamond will direct more light through the crown to maximize brilliance, sparkle and fire.

In 2005 the GIA also unveiled a diamond cut grading system for standard round brilliants in the D to Z color range. This system assigns an overall diamond cut grade ranging from Excellent to Poor.