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The Gemological Institute of America (GIA) has devised a set of guidelines to grade diamonds by color. Grading is done in a consistent manner using control diamonds that are compared to the diamond being examined.
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Carat-Weight:
The earliest gem traders measured their stones by comparing their weight to the weight of the pods from the carob, or locust tree. These tiny carob seeds are remarkably consistent in weight, and this is where the term ‘carat’ is derived from.
The metric carat, which equals 0.20 gram or 1/142 of an ounce, is the standard unit of weight for diamonds and most other gems. One carat is divided into 100 "points." A diamond weighing 50 points = 0.5 carat (ct.), and is expressed as one-half carat. The carat-weight of a diamond is the most obvious characteristic of a diamond, and larger stones are often prized due to their rarity. It must be remembered, however, that the weight has no bearing on quality, and two diamonds of comparable weight may differ drastically in price due to dissimilarities in cut, color, and clarity. Diamonds of the same diameter may vary in actual carat weight depending on variations in their proportions. All other aspects being equal, the larger the carat weight, the more valuable the diamond is.
One Grain or "a one grainer":
25 points or 1/4ct.; A diamond of approximately 1ct. would be called a 4 grainer, one ~ 1.5ct. would be a 6 grainer, and a 0.5ct. stone a 2 grainer.
Melee:
Diamonds weighing less than 1/5ct. (less than 20 points). Stones this size are set using various techniques as groupings in jewelry.
Weight representation:
In the U.S., the Federal Trade Commission requires that diamonds be weighed to one-thousandth of one carat (0.001ct.), but that the final weight can be rounded to the nearest half point (0.005ct.). This means that a diamond weighing 0.995ct. can be represented to you as a 1.00ct. stone. However, international convention is more strict, and a weight can only be rounded to the next higher point (0.01ct.) if the actual weight reaches 9/10ths (0.009ct.) of a point. For example, the 0.995ct. diamond must be represented as a 0.99ct. stone, and to be represented as a 1.00ct. diamond the stone must weight 0.999ct. before it can be rounded up to 1.00ct.
Value per carat:
increases with carat size, because larger rough diamonds occur less frequently. In other words, 2 half-carat diamonds taken together will not cost as much as 1 one-carat diamond, as the one-carat stone is more rare. Also, a premium is added to diamond prices as they reach and exceed each 1/4ct. increment in weight.
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Color:
The color grade of a diamond refers to how closely its body color approaches colorlessness. The best color for a colorless diamond is, in fact, an absence of color. A truly colorless stone will carry a premium price, and the larger that diamond, the greater the premium per carat.
Most diamonds have at least a trace of yellow, brown, or gray body color. Though a lot of diamonds will appear to be colorless, most actually possess subtle shade differences. These variances in color are due to traces of elements such as nitrogen and boron that become assimilated into a diamond’s atomic structure during the original formation.
Most commonly, a diamond will have a hint of yellow or brown due to traces of nitrogen.
Diamonds with increasing shades of brown and yellow are referred to as champagne diamonds, and diamonds with exceptional color, such as pink, red, green, blue, and amber, are called "Fancies." With the exception of some natural fancy colors, such as blue, pink, purple, or red, the colorless grade is the most rare. Colors of high saturation such as red and green, which have no modifying secondary colors, are very rare indeed and command world record prices.
The rarest and most prized diamonds are those with no hint of color at all whose beauty is entirely dependent upon their optical properties. Such stones reflect all of the colors of the rainbow, and their value is great. All things being equal, other than the fancy colors, the closer a diamond is to colorless, the more valuable it is.
The color of the stone can be determined by using a GIA (Gemological Institute of America) Certified set of master stones and/or the Colorimeter, a computer which accurately grades the color of a polished diamond.
Factors Affecting Color:
Other factors which can affect color include fluorescence, color enhancements such as high pressure, high temperature treatment (HPHT) and irradiation.
Fluorescence:
produced by ultraviolet light from the sun, by black lighting or other long-wavelength UV source, occurs in an estimated 35% of gem grade diamonds. (Virtually all diamonds fluoresce when exposed to X-rays, and this forms the basis for their identification and collection at mining sites.) The UV light excites electrons in the diamond crystal, which then release this absorbed energy in the form of visible light, producing a blue, or sometimes other color, of faint to very strong intensity. Once the light source is removed however, the fluorescence is no longer observed. If, in rare situations, light emission continues for a period after the exciting light has been turned off, the phenomenon is called phosphorescence.
Blue Fluorescence:
if strong or very strong, may alter the perceived color of a diamond in a negative or positive way. For example, stones in the colorless/near colorless ranges (D-H) may appear milky or oily, detracting from their appearance. On the other hand, diamonds in the lower, more yellow color ranges (I and lower) may appear to have less yellow color due to the fluorescence, adding positively to their appearance. Thus, the trade will slightly discount prices of diamonds in the former category, while sometimes adding a very slight premium to those in the latter.
The presence and color (most frequently blue, but can be any color) of fluorescence and its intensity (none/inert, faint, medium, strong, very strong) are indicated on all GIA and AGS grading reports.
Irradiation:
of diamonds using a nuclear reactor or linear accelerator, sometimes in combination with heating, is used to produce a variety of different fancy colored stones.
High Pressure/High Temperature (HPHT) treatment:
is a process developed by General Electric whereby type IIa diamonds of low color (N-O) or even fancy brown color, can be converted to colorless/near colorless (D-H) forms by an annealing process involving high pressure and temperature.
There is some concern that these color-enhanced stones may come to market undetected, but new research has shown that they can be largely detected using expensive and sophisticated equipment. Similarly, the HPHT technique has also been applied to type Ia brown diamonds by several companies, yielding fancy yellowish green and greenish yellow colors. Pink and blue colors have also been produced by the same technique.
The Color Grading Scale: ranges from totally colorless to fancy yellow. The differences between one grade and another are very subtle, as can be seen by the number of grades within any one category.
Diamonds are graded for color only as loose, unmounted stones in the inverted position (table down, pavilion up), and under very specific conditions of lighting and background, and also the distance and angle of the observer in relation to the stone. A color scale has been devised by the Gem Trade Lab. of the Gemological Institute of America (GIA) divided alphabetically from D (the absence of color) to Z (fancy yellow color), and is recognized world-wide. Each letter grade represents a small range of color, and not just a single color. Diamonds are color graded by being compared to a set of master stones.
DEF (Colorless)
GHI Near colorless; not distinguishable from colorless when mounted
J Very, very faint yellow
KLM Faint yellow. Color distinguishable to the trained eye.
NOPQ Very light yellow to intense yellow color.
RSTUVWXYZ Increasingly more intense color, gradually ranging into browns.
Effect of Color on Price:
In general, as the amount of yellow in the stone increases, the value of a diamond decreases, that is until the fancy color grades are reached, and then the value goes up again.
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Cut:
Regardless of the size or shape of a diamond, the cut will be the ultimate determinant of a diamond’s brilliance and fire. Most round, brilliant-cut or fancy-shaped diamonds have fifty-eight delicately angled flat surfaces, called facets. The placement of each facet is intrinsic to the brilliance and beauty of the stone; if the cut is too deep, light will escape through the opposite side of the pavilion, or bottom, of the diamond before it can be reflected. Similarly, a shallow cut will allow light to pass directly through the pavilion. The cut of a diamond is graded on the basis of how well the diamond handles the light coming into it from above.
The proportions and finish of a polished diamond determine its final face-up beauty. A well proportioned diamond will return most of the light back to the eye for maximum brilliance and dispersion.
Sparkle is created by the facet placement and the surface polish.
How a diamond handles light:
1. When a diamond is cut to good proportions, light is reflected from one facet to another and then dispersed through the top of the stone.
2. If the cut of the diamond is too deep, some light escapes through the opposite side of the pavilion.
3. If the cut is too shallow, light escapes through the pavilion before it can be reflected, thus reducing brilliancy.
Brilliance:
is the total light reflected to the eye from both surface and internal reflections.
Dispersion:
is the separation of white light into its spectral (rainbow) colors and is caused by its refraction, which bends each wavelength of light a little differently to cause separation into individual colors.
Scintillation:
is the flashing or sparkling of light which occurs when either the diamond or light source are moved.
Finish:
which includes polish and symmetry, is also an important contributor to a diamond's beauty.
Polish:
features largely describe the condition of the surface of the stone, and include abrasions of the facet junctions and the culet, rough and bearded girdles, nicks and pits, and scratches and polishing marks if they do not affect the clarity grade. You will remember some these elements were also listed as blemishes in the clarity-grading scheme. However, if the blemishes are not serious enough to affect the clarity grade, they are included under polish as part of the finish grade.
Symmetry:
characteristics include many features of poor and/or careless workmanship such as an off-center table or culet, a table not a regular octagon shape, an out-of-round or wavy girdle, non-parallel girdle and table, facets which don't meet or point properly or are misaligned between the crown and pavilion, and naturals and extra facets which are not graded under clarity.
Proportion:
The full realization of the potential of a diamond's brilliance, dispersion and scintillation and getting the proper balance between these three elements is achieved mainly through proper cut proportions. The important proportions are:
1. Table size
2. Crown height and angle (angle of the bezel facets with the girdle)
3. Pavilion depth and angle (angle of the pavilion facets with the girdle)
4. Girdle thickness
5. Culet size
Make:
This is a trade term used to describe how closely the proportions of a brilliant cut diamond are to an "ideal". For stones approaching these proportions a diamond is said to be of good or fine make, and for those with problems of proportion are referred to as being of fair or poor make.
Effect of Cut Quality on Price:
Cut quality commands a premium for two reasons. First, you are paying for a highly skilled diamond cutter's time, and it can take many hours to get all the proportions and angles to fall within the Ideal or SuperIdeal ranges. Second, more of the diamond is lost in the cutting process, because the goal is not to cut the heaviest diamond, but the best performing diamond.
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Grading-Clarity:
describes the clearness or purity of a diamond. This is determined by the number, size, nature, and location of the internal (inclusions) and external (blemishes) imperfections.
Nitrogen and other elements trapped within a stone during its formation effect the ultimate color, and minerals embedded in a diamond during crystallization will influence the stone’s clarity.
These natural characteristics, called ‘inclusions’, make each stone unique; they’re nature’s way of personalizing each diamond.
Gemologists even use these inclusions to determine a diamond’s age. The fewer the inclusions, however, the more rare and precious the stone will be considered because nothing will interfere with the passage and reflection of light. The number, color, size, and position of any inclusions will specify a diamond’s clarity, but most will not endanger its durability. To establish a diamond’s clarity, it must be examined by a trained eye under a 10x magnification; the fewer the inclusions, the more valuable the diamond will be.
Be aware!
Two methods used to enhance the clarity grade given to a diamond are laser drilling and fracture filling.
Laser drilling is the process in which a laser is used to drill a tiny hole into a diamond and the black of the imperfection is then removed.
The second treatment is fracture filling. This method is not permanent and therefore not recommended.
BLEMISHES:
Abrasions:
tiny nicks on facet junctions or the culet; caused by wear or coming in contact with other diamonds.
Extra Facets:
small facets placed to remove imperfections; not part of the cutting style. (Not to be confused with Added Facets which are added symmetrically and are part of the cutting style).
Naturals:
part of the original crystal surface remaining on the polished stone, frequently in the area of the girdle.
Polish Lines and Marks:
tiny parallel lines or surface clouding left by irregular polishing or excessive heating during polishing, respectively.
Rough Girdle:
a grainy or pitted girdle surface usually caused by poor workmanship.
Surface Graining:
structural irregularities in crystal growth; may appear as faint lines, causing grooved or wavy surfaces and often cross facet junctions.
INCLUSIONS:
Bearding:
tiny feathers extending inward from a bruted girdle surface. (Bruting is the process of rubbing two diamonds together to achieve the rounded shape of the diamond).
Cavities and Chips:
large/deep openings, and small/shallow openings in the diamond's surface, respectively.
Clouds:
hazy or milky areas of many very small, usually crystalline inclusions.
Feathers:
cleavages or fractures often white and feathery in appearance. (There are 4 cleavage planes in diamond, which run in octahedral directions. Fractures are breaks along planes other than cleavage planes and may alternate with them to form step-like feathers).
Included Crystals:
mineral crystals, such as garnet or peridot, contained inside a diamond.
Indented Naturals:
natural rough surfaces that penetrate the stone and may distort the girdle outline.
Internal Graining:
regions of irregular crystal growth that may appear as milky or colored lines or streaks, or may be reflective.
Laser Drill Holes:
a tiny tube made by a laser; the surface opening may resemble a pit, while the tube usually resembles a needle.
Needles:
needle-shaped included crystals.
Pinpoints:
areas of minute, dot-like inclusions.
Twinning Wisps:
cloudy areas produced by distorted crystal growth.
Clarity::
All clarity grading is performed at 10-times magnification using a hand loupe or gemological microscope under both artificial daylight and darkfield illumination conditions. Reflected light is used to detect and evaluate blemishes and darkfield light for inclusions. It's the face-up view that usually sets the clarity grade, however the face-up, pavilion and table-to-culet views are all taken into consideration during grading.
Clarity grades are largely determined by the collective visual appearance that a stone's inclusions exhibit in relationship to the size and shape of the stone. It is the consideration of the size, position, number, color/contrast and nature of these inclusions, which leads to the final clarity grade.
Clarity Symbol Meaning Definition:
F Flawless Free from all blemishes, or inclusions.
IF Internally Flawless No inclusions visible at 10x magnification.
VVS1 Very Very Slightly Included #1 Inclusions that are extremely difficult to detect at 10x.
VVS2 Very Very Slightly Included #2 Inclusions that are very difficult to detect at 10x.
VS1 Very Slightly Included #1 Minor inclusions, difficult to detect at 10x.
VS2 Very Slightly Included #2 Minor inclusions, somewhat difficult to detect at 10x.
SI1 Slightly Included #1 Noticeable inclusions, easy to detect at 10x.
SI2 Slightly Included #2 Noticeable inclusions, very easy to detect at 10x.
SI3Slightly Included #3 Some inclusions may be seen with the unaided eye.
I1 Included #1 Obvious inclusions. Somewhat easy to detect with the unaided eye.
I2 Included #2 Obvious inclusions. Easy to locate with the unaided eye.
I3 Included #3 Obvious inclusions. Very easy to detect with the unaided eye.
The above clarity grading scheme is in accordance with the GIA (Gemological Institute of America).
In addition to the traditional Round Brilliant shape, the most common Fancy Shapes are: Oval, Pear, Marquise, Princess, Radiant, Emerald and Heart. See the chart below!
Less common shapes include Trillion (or Triangular), Flanders and Asscher.
Percentage are two critical measurements for loose diamonds. The angles of cut will affect a diamond's natural ability to both reflect and refract light - that is, the diamond's ability to virtually explode with brilliance. If a diamond is cut too deep, light leaks out the sides, brilliance is lost and the center of the diamond will appear to be dark. If a diamond is cut too shallow, light leaks out the bottom and the diamond appears watery, glassy and dark. If the proportions are good, the sides and bottom of the diamond will all reflect their light back out the top of the diamond, which creates the brilliance diamonds are famous for.
The TABLE percent is the width of the top facet of the crown compared to the diamond's total width. Table width between 53% and 64% are considered Preferred in Round Brilliant Cut diamonds. The DEPTH percent is the total depth or height of the diamond compared to the total width. Depth measurements between 58% and 63% are considered Preferred in Round Brilliant Cut Diamonds. The Preferred range for these measurements will change as the shape changes, i.e. a Marquise cut diamond will have different ranges for Premium Table and Depth percentages.
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