Diamond: Encyclopedia II - Diamond - Gemological characteristics

Diamond - Gemological characteristics

The use of diamonds as gemstones of decorative value is the most familiar use to most people today, and is also the earliest use, with decorative use of diamonds stretching back into antiquity. The dispersion of white light into a rainbow of colors, known in the trade as fire, is the other primary characteristic of gem diamonds, and has been highly prized throughout history. Over time, especially since around 1900, experts in the field of gemology have developed methods of characterizing diamonds and other gemstones based on the characteristics most important to their value as a gem. Four characteristics, known informally as the four Cs, are now commonly used as the basic descriptors of diamonds: these are carat, clarity, color, and cut.

Most gem diamonds are traded on the wholesale market based on single values for each of the four Cs; for example knowing that a diamond is rated as 1.5 carats, VS2 clarity, F color, excellent cut, is enough to reasonably establish an expected price range. More detailed information from within each characteristic can then be used to determine actual market value for individual stones. Consumers who purchase individual diamonds are often advised to use the four Cs to pick the diamond that is "right" for them; to these is sometimes added the "fifth C" of cost.

Other characteristics not described by the four Cs can and do influence the value or appearance of a gem diamond. These characteristics include physical characteristics such as the presence of fluorescence, as well as data on a diamond's history including its source and which gemological institute performed evaluation services on the diamond. Cleanliness also dramatically affects a diamond's beauty.

There are four major gemological associations which "certify" diamonds: that is, define the four Cs of a diamond. While carat weight and cut angles are mathematically defined, the clarity and color are judged by the trained human eye and are therefore open to slight variance in interpretation.

• Gemological Institute of America (GIA) was the first laboratory to issue modern diamond reports, and holds the highest reputation amongst gemologists for its consistent, conservative grading.
• American Gemological Society (AGS) is not as widely recognized nor as old as the GIA, but garners an equally high reputation.
• International Gemological Laboratory (IGL) is a generally respected laboratory but suffers from a negative industry reputation for its grading practices, which are perceived by critics as being either less conservative or less consistent than the GIA and AGS.
• European Gemological Laboratory (EGI) has a similar reputation to the IGL.

Diamond - Carat

The carat weight measures the mass of a diamond. One carat is defined as exactly 200 milligrams (about 0.007 ounce). The point unit—equal to one one-hundredth of a carat (0.01 carat, or 2 mg)—is commonly used for diamonds of less than one carat. All else being equal, the value of a diamond increases exponentially in relation to carat weight, since larger diamonds are both rarer and more desirable for use as gemstones. A review of comparable diamonds available for purchase in September 2005 demonstrates this effect (approximate prices for round cut, G color, VS2 diamonds with "1A" cut grade, as listed on http://www.pricescope.com):

The price per carat does not increase smoothly with increasing size. Instead, there are sharp jumps around milestone carat weights, as demand is much higher for diamonds weighing just more than a milestone than for those weighing just less. As an example, a 0.95 carat diamond may have a significantly lower price per carat than a comparable 1.05 carat diamond, because of differences in demand.

A weekly price list published by Rapaport of New York, of diamond prices per carat, for different diamond cuts, clarity and weights, is currently considered the de-facto retail price baseline. Jewelers often trade diamonds at negotiated discounts off the Rapaport price (e.g., "R -3%").

In the wholesale trade of gem diamonds, carat is often used in denominating lots of diamonds for sale. For example, a buyer may place an order for 100 carats of 0.5 carat, D–F, VS2-SI1, excellent cut diamonds, indicating he wishes to purchase 200 diamonds (100 carats total mass) of those approximate characteristics. Because of this, diamond prices (particularly among wholesalers and other industry professionals) are often quoted per carat, rather than per stone.

Total carat weight (t.c.w.) is a phrase used to describe the total mass of diamonds or other gemstone in a piece of jewelry, when more than one gemstone is used. Diamond solitaire earrings, for example, are usually quoted in t.c.w. when placed for sale, indicating the mass of the diamonds in both earrings and not each individual diamond. T.c.w. is also widely used for diamond necklaces, bracelets and other similar jewelry pieces.

Diamond - Clarity

Clarity is a measure of internal defects of a diamond called inclusions. Inclusions may be crystals of a foreign material or another diamond crystal, or structural imperfections such as tiny cracks that can appear whitish or cloudy. The number, size, color, relative location, orientation, and visibility of inclusions can all affect the relative clarity of a diamond. The Gemological Institute of America (GIA) and others have developed systems to grade clarity, which are generally based on those inclusions which are visible to a trained professional when a diamond is viewed from above, under 10x magnification.

Diamonds become increasingly rare when considering higher clarity gradings. Only about 20 percent of all diamonds mined have a clarity rating high enough for the diamond to be considered appropriate for use as a gemstone; the other 80 percent are relegated to industrial use. Of that top 20 percent, a significant portion contains an inclusion or inclusions that are visible to the naked eye upon close inspection. Those that do not have a visible inclusion are known as "eye-clean" and are preferred by most buyers, although visible inclusions can sometimes be hidden under the setting in a piece of jewelry.

Most inclusions present in gem-quality diamonds do not affect the diamonds' performance or structural integrity. However, large clouds can affect a diamond's ability to transmit and scatter light. Large cracks close to or breaking the surface may reduce a diamond's resistance to fracture.

Diamonds are graded by the major societies on a scale ranging from Flawless to Imperfect. (see the main article for more detail)

Diamond - Color

A chemically pure and structurally perfect diamond is perfectly transparent with no hue, or color. However, in reality almost no gem-sized natural diamonds are absolutely perfect. The color of a diamond may be affected by chemical impurities and/or structural defects in the crystal lattice. Depending on the hue and intensity of a diamond's coloration, a diamond's color can either detract from or enhance its value. For example, most white diamonds are discounted in price as more yellow hue is detectable, while intense pink or blue diamonds (such as the Hope Diamond) can be dramatically more valuable.

Most diamonds used as gemstones are basically transparent with little tint, or white diamonds. The most common impurity, nitrogen, replaces a small proportion of carbon atoms in a diamond's structure and causes a yellowish to brownish tint. This effect is present in almost all white diamonds; in only the rarest diamonds is the coloration due to this effect undetectable. The GIA has developed a rating system for color in white diamonds, from "D" to "Z" (with D being "colorless" and Z having a bright yellow coloration), which has been widely adopted in the industry and is universally recognized, superseding several older systems once used in different countries. The system uses a benchmark set of either natural diamonds of known color grade, or precision-crafted cubic zirconia; test lighting conditions are also standardized and carefully controlled. Diamonds with higher color grades are rarer, in higher demand, and therefore more expensive, than lower color grades. Oddly enough, diamonds graded Z are also rare, and the bright yellow color is also highly valued. Diamonds graded D-F are considered "colorless", G-J are considered "near-colorless", K-M are "slightly colored". N-Y are usually appear light yellow or brown.

In contrast to yellow or brown hues, diamonds of other colors are much rarer and more valuable. While even a pale pink or blue hue may increase the value of a diamond, more intense coloration is usually considered more desirable and commands the highest prices. A variety of impurities and structural imperfections cause different colors in diamonds, including yellow, pink, blue, red, green, brown, and other hues. Diamonds with unusual or intense coloration are sometimes labeled "fancy" by the diamond industry. Intense yellow coloration is considered one of the fancy colors, and is separate from the color grades of white diamonds. Gemologists have developed rating systems for fancy colored diamonds, but they are not in common use because of the relative rarity of colored diamonds.

Diamond - Cut

Diamond cutting is the art and science of creating a gem-quality diamond out of mined rough. The cut of a diamond describes the manner in which a diamond has been shaped and polished from its beginning form as a rough stone to its final gem proportions. The cut of a diamond describes the quality of workmanship and the angles to which a diamond is cut. Often diamond cut is confused with "shape."

There are mathematical guidelines for the angles and length ratios at which the diamond is supposed to cut at in order to reflect the maximum amount of light. Round brilliant diamonds, the most common, are guided by these specific guidelines, though fancy cut stones are not able to be as accurately guided by mathematical specifics.

The techniques for cutting diamonds have been developed over hundreds of years, with perhaps the greatest achievements made in 1919 by mathematician and gem enthusiast Marcel Tolkowsky. He developed the round brilliant cut by calculating the ideal shape to return and scatter light when a diamond is viewed from above. The modern round brilliant has 57 facets (polished faces), counting 33 on the crown (the top half), and 24 on the pavilion (the lower half). The girdle is the thin unpolished middle. The function of the crown is to diffuse light into various colors and the pavilion's function to reflect light back through the top of the diamond.

Tolkowsky defines the ideal dimensions to have:

• Table percentage (table diameter divided by overall diameter) = 53%
• Depth percentage (Overall depth divided by the overall diameter) = 59.3%
• Pavilion Angle (Angle between the girdle and the pavilion) = 40.75°
• Crown Angle (Angle between the girdle and the crown) = 34.5°
• Pavilion Depth (Depth of pavilion divided by overall diameter) = 43.1%
• Crown Depth (Depth of crown divided by crown diameter) = 16.2%

The culet is the tiny point at the bottom of the diamond. This should be a negligible diameter, otherwise light leaks out of the bottom. Tolkowsky's ideal dimensions did not include a girdle. However, a thin girdle is required in reality in order to prevent the diamond from easily chipping in the setting. A normal girdle should be about 1%–2% of the overall diameter.

The further the diamond's characteristics are from Tolkowsky's ideal, the less light will be reflected. However, there is a small range in which the diamond can be considered "ideal." Today, because of the relative importance of carat weight in society, many diamonds are often intentionally cut poorly to increase carat weight. There is a financial premium for a diamond that weighs the magical 1.0 carat, so often the girdle is made thicker or the depth is increased. Neither of the these tactics make the diamond appear any bigger, but it also greatly reduces the sparkle of the diamond. So a poorly cut 1.0 carat diamond may have the same diameter and appear as large as a 0.85 carat diamond. The depth percentage is the overall quickest indication of the quality of the cut of a round brilliant. "Ideal" round brilliant diamonds should not have a depth percentage greater than 62.5%. Another quick indication is the overall diameter. Typically a round brilliant 1.0 carat diamond should have a diameter of about 6.5 mm. Mathematically, the diameter in millimeters of a round brilliant should approximately equal 6.5 times the cube root of carat weight, or 11.1 times the cube root of gram weight.

Diamonds do not show all of their beauty as rough stones; instead, they must be cut and polished to exhibit the characteristic fire and brilliance that diamond gemstones are known for. Diamonds are cut into a variety of shapes that are generally designed to accentuate these features.

Diamonds which are not cut to the specifications of Tolkowsky's round brilliant shape (or subsequent variations) are known as "fancy cuts." Popular fancy cuts include the baguette (from the French, resembling a loaf of bread), marquise, princess (square outline), heart, briolette (a form of the rose cut), and pear cuts. Generally speaking, these "fancy cuts" are not held to the same strict standards as Tolkowsky-derived round brilliants and there are less specific mathematical guidelines of angles which determine a well-cut stone. Cuts are influenced heavily by fashion: the baguette cut—which accentuates a diamond's luster and downplays its fire—was all the rage during the Art Deco period, whereas the princess cut—which accentuates a diamond's fire rather than its luster—is currently gaining popularity. The princess cut is also popular amongst diamond cutters: of all the cuts, it wastes the least of the original crystal. The past decades have seen the development of new diamond cuts, often based on a modification of an existing cut. Some of these include extra facets. These newly developed cuts are viewed by many as more of an attempt at brand differentiation by diamond sellers, than actual improvements to the state of the art.

The quality of a diamond's cut is widely considered the most important of the four Cs in determining the beauty of a diamond; indeed, it is commonly acknowledged that a well-cut diamond can appear to be of greater carat weight, and have clarity and color appear to be of better grade than they actually are. The skill with which a diamond is cut determines its ability to reflect and refract light.

In addition to carrying the most importance to a diamond's quality as a gemstone, the cut is also the most difficult to quantitatively judge. A number of factors, including proportion, symmetry, and the relative angles of various facets, are determined by the quality of the cut and can affect the performance of a diamond. A poorly cut diamond with facets cut only a few degrees out of alignment can result in a poorly performing stone. For a round brilliant cut, there is a balance between "brilliance" and "fire." When a diamond is cut for too much "fire," it looks like a cubic zirconia, which gives off much more "fire" than real diamond. A well executed round brilliant cut should reflect most light out from the tabletop and make the diamond appear white when viewed from the top. An inferior cut will produce a stone that appears dark at the center and in some extreme cases the ring settings may show through the top of the diamond as shadows.

Several different theories on the "ideal" proportions of a diamond have been and continue to be advocated by professional gemologists. Recently, there has been a shift away from grading cut by the use of various angles and proportions toward measuring the performance of a cut stone. A number of specially modified viewers have been developed toward this end. One result of this trend is the rise of the phrase "hearts and arrows," describing a characteristic pattern observable on stones exhibiting high symmetry. Hearts and arrows diamonds trade at a 10 to 20 percent premium to otherwise comparable diamonds.

The process of shaping a rough diamond into a polished gemstone is both an art and a science. The choice of cut is often decided by the original shape of the rough stone, location of the inclusions and flaws to be eliminated, the preservation of the weight, popularity of certain shapes amongst consumers and many other considerations. The round brilliant cut is preferred when the crystal is an octahedron, as often two stones may be cut from one such crystal. Oddly shaped crystals such as macles are more likely to be cut in a fancy cut—that is, a cut other than the round brilliant—which the particular crystal shape lends itself to.

Even with modern techniques, the cutting and polishing of a diamond crystal always results in a dramatic loss of weight; rarely is it less than 50%. Sometimes the cutters compromise and accept lesser proportions and symmetry in order to avoid inclusions or to preserve the carat rating. Since the per carat price of diamond shifts around key milestones (such as 1.00 carat), many one-carat diamonds are the result of compromising "Cut" for "Carat." Some jewelry experts advise consumers to buy a 0.99 carat diamond for its better price or buy a 1.10 carat diamond for its better cut, avoiding a 1.00 carat diamond which is more likely to be a poorly cut stone.

Diamond - Cleaning

Although it is not one of the four Cs, cleanliness affects a diamond's beauty as much as any of the four Cs. A clean diamond is more brilliant and fiery than the same diamond when it is "dirty". Dirt or grease on the top of a diamond reduces its luster. Water, dirt, or grease on the bottom of a diamond interferes with the diamond's brilliance and fire. Even a thin film absorbs some light that could have been reflected to the person looking at the diamond. Colored dye or smudges can affect the perceived color of a diamond. Historically, some jewelers' stones were misgraded because of smudges on the girdle, or dye on the culet. Current practice is to thoroughly clean a diamond before grading its color.

Maintaining a clean diamond can sometimes be difficult, as jewelry settings can obstruct cleaning efforts, and oils, grease, and other hydrophobic materials adhere well to a diamond's surface. Some jewelers provide their customers with ammonia-based cleaning kits; ultrasonic cleaners are also popular.

Cleanliness does not affect the diamond's market value, as any competent jeweler will clean the diamond before offering it for sale. However, cleanliness might reflect a diamond's sentimental value: some jewelers have noted a correlation between ring cleanliness and marriage quality [1].

Adapted from the Wikipedia article "Gemological characteristics", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki