Diamond is the only gem made of a single element: It is typically about 99.95 percent carbon. The other 0.05 percent can include one or more trace elements, which are atoms that aren’t part of the diamond’s essential chemistry. Some trace elements can influence its color or crystal shape. Diamond forms under high temperature and pressure conditions that exist only about 100 miles beneath the earth’s surface. Diamond’s carbon atoms are bonded in essentially the same way in all directions. Another mineral, graphite, also contains only carbon, but its formation process and crystal structure are very different. Graphite is so soft that you can write with it, while diamond is so hard that you can only scratch it with another diamond.

 photo credit :  Jeff Scovil, courtesy Barker & Co, courtesy of GIA.edu

photo credit: Jeff Scovil, courtesy Barker & Co, courtesy of GIA.edu

The 4Cs describe the individual qualities of a diamond, and the value of an individual diamond is based on these qualities. The terms that people use to discuss the 4Cs have become part of an international language that jewelry professionals can use to describe and evaluate individual diamonds.

A diamond’s value is often affected by the rarity of one or more of the 4Cs. Colorless diamonds are scarce—most diamonds have tints of yellow or brown. So a colorless diamond rates higher on the color grading scale than a diamond that is light yellow. Value and rarity are related: In this case a colorless diamond is more rare and more valuable than one with a slight yellow color. The same relationship between rarity and value exists for clarity, cut, and carat weight.
 
Today, the descriptions of each of the 4Cs are more precise than those applied to almost any other consumer product. And they have a long history. Three of them—color, clarity, and carat weight—were the basis for the first diamond grading system established in India over 2,000 years ago.

 photo courtesy of  GIA.edu

photo courtesy of GIA.edu

COLOR
Subtle differences in color can dramatically af­fect diamond value. Two diamonds of the same clarity, weight, and cut can differ in value based on color alone. Even the slightest hint of color can make a dramatic difference in value.

 photo courtesy of  GIA.edu

photo courtesy of GIA.edu

Diamonds come in many colors. Diamonds that range from colorless to light yellow and brown fall within the normal color range. Within that range, colorless diamonds are the most rare, so they’re the most valuable. They set the standard for grading and pricing other diamonds in the normal color range.

At the GIA Laboratory, diamonds are color graded under controlled conditions by comparing them to round brilliant diamonds of known color, called masterstones.

Many diamonds emit a visible light called fluorescence when they’re exposed to ultraviolet (UV) radiation. Although invisible to the human eye, UV radiation is everywhere. Sunlight contains it. Fluorescent lights emit it, too. Under the right conditions, you can see fluorescence in about 35 percent of gem diamonds.
 
Blue is the most common fluorescent color in gem-quality diamonds. In rare instances, fluorescence can be white, yellow, orange, or many other colors.
 
Strong blue fluorescence can make a light yellow diamond look closer to colorless in sunlight. Blue and yellow are color opposites and tend to cancel each other out, so blue fluorescence masks the yellow color. If the fluorescence is too strong it can make the stone look cloudy or “oily,” which can lower the value of the diamond.


 photo courtesy of  GIA.edu

photo courtesy of GIA.edu

CLARITY

Few things in nature are absolutely perfect. This is as true of diamonds as anything else. Diamonds have internal features, called inclusions, and surface irregularities, called blemishes. Together, they’re called clarity characteristics. Clarity is the relative absence of inclusions and blemishes.
 
Among other things, blemishes include scratches and nicks on a diamond’s surface. Inclusions are generally on the inside, and some might break the surface of the stone. Sometimes, tiny diamond or other mineral crystals are trapped inside a diamond when it forms. Depending on where they’re located, they might remain after the stone has been cut and polished, and they can affect a diamond’s appearance.
 
Clarity characteristics might have a negative influence on a diamond’s value, but they can have positive effects as well. For one thing, they help gemologists separate diamond from imitations. (This is easier with included diamonds than with flawless ones.) And because no two diamonds have exactly the same inclusions, they can help identify individual stones. They can also provide scientists with valuable information about how diamonds form.

 photo courtesy of  GIA.com

photo courtesy of GIA.com

Flawless diamonds are very rare—so rare, in fact, that it’s possible to spend a lifetime in the jewelry industry without ever seeing one, and they command top prices.
 
At the other end of the scale are diamonds with inclusions that can be easily seen by the unaided eye. Between the two extremes are diamonds with inclusions visible only under 10X magnification. Stones in the middle range make up the bulk of the retail market.

There are 11 clarity grades in the GIA clarity grading system. They are Flawless, Internally Flawless, two categories of Very, Very Slightly Included, two categories of Slightly Included, and three categories of Included.
The effect of a clarity characteristic on the clarity grade is based on its size, number, position, nature, and color or relief.

 photo courtesy of  GIA.edu

photo courtesy of GIA.edu

A beautifully finished diamond is dazzling, with every facet displaying the craftsman’s skill and care. When a diamond interacts with light, every angle and every facet affects the amount of light returned to the eye. This is what gives it its face-up appearance.

A diamond’s proportions determine how light performs when it enters the diamond. If light enters through the crown and goes out through the pavilion, the diamond will look dark and unattractive.  Diamonds with different proportions and good polish make better use of the light, and will be bright, colorful, and scintillating.

As a general rule, the higher the cut grade, the brighter the diamond. Under fluorescent lighting, these diamonds (left to right) display high, moderate, and low brightness.
The term “cut” also can describe a fashioned diamond’s shape. Shapes other than the standard round brilliant are called fancy cuts. They’re sometimes called fancy shapes or fancies. Fancy shapes also have names of their own, based on their shapes. The best known are the marquise, princess, pear, oval, heart, and emerald cut.

 Photo Courtesy Lazare Kaplan Diamonds via  GIA.edu

Photo Courtesy Lazare Kaplan Diamonds via GIA.edu


CARAT WEIGHT
Many goods are sold by weight—by the kilogram, ounce, pound, or ton. Even people who have never bought a diamond are used to the idea that weight and price are related. They understand that a larger diamond is probably more valuable than a smaller one. But there are two things that often surprise people when they start learning about diamonds and carat weight.
 
The first is the precision with which diamonds are weighed. Diamond weights are stated in metric carats, abbreviated “ct.” One metric carat is two-tenths (0.2) of a gram—just over seven thousandths (0.007) of an ounce. One ounce contains almost 142 carats. A small paper clip weighs about a carat.

The metric carat is divided into 100 points. A point is one hundredth of a carat.
 
Diamonds are weighed to a thousandth (0.001) of a carat and then rounded to the nearest hundredth, or point. Fractions of a carat can mean price differences of hundreds—even thousands—of dollars, depending on diamond quality.
 
Over a carat, diamond weights are usually expressed in carats and decimals. A 1.03-carat stone, for example, would be described as “one point oh three carats,” or “one oh three.” Weights for diamonds that weigh under a carat are usually stated in points. A diamond that weighs 0.83 carat is said to weigh “eighty-three points,” or called an “eighty-three pointer.”

The relationship between rarity, weight, and value can be surprising. People know that a pound of sugar costs twice as much as a half-pound of sugar. But diamonds aren’t a commodity like sugar. Their price depends on a number of variables—weight is just one of them. So it’s not always easy to understand, or explain, why a 1-carat diamond is worth, say, $6,000, while a 2-carat diamond of similar quality might be worth $15,000.

It’s really a simple concept: Large diamonds are more rare than small diamonds. The more scarce something is, the more it is worth. So a larger stone doesn’t just cost more. It also costs more per carat. A 1-carat diamond weighs the same as four 0.25-carat diamonds. But even if all the other quality factors are equal, the larger diamond is worth much more than the sum of the four smaller diamonds.



All text and photos are quoted from gia.edu  article "diamond quality factors." We highly recommend GIA's excellent educational resources.