The Diamond Exchange by Hilton
Cut, Clarity, Color & Carat
The 4-C's namely, Cut, Clarity, Color and Carat Weight help to categorize and grade the diamond using universal terminology which determines also the price and value of the diamond. At the same time familiarity with the 4 C's helps individuals to better understand the diamond's natural characteristics and thus to decide knowledgeably which ombination is the most suitable and most desirable for them . 
How to buy a diamond - the 4 C's
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Proportions and cut determine the brilliance of a diamond. In order to maximize this brilliance, the diamond cutter must place each of the diamond's facets, which act as light-dispersing mirrors, in exact geometric relation to one another. On a classic Round Brilliant cut diamond, fifty eight (!) facets must be precisely aligned. Few diamonds are cut to exacting standards since diamond cutters try to maximize their returns on the raw material by leaving the stone as large as possible with minimum waste. As a result, the proportions, symmetry and perfection of the cut and shape may be delegated to play a secondary role. The result is usually a compromise between profit (e.g., size) and beauty (perfection of cut, proportions and shape).  
The most important criteria of the Cut is the ratio of the depth to its diameter ( i.e. Depth/Diameter). In order for the diamond to be considered within the "Ideal" tolerance bracket the ratio should be between 58/100 and 62/100 or, "58%" and "62%" respectively (see illustration). The measurements are taken in millimeters through the use of a Leveridge Gauge or Micrometer "Table" diameter percentage ( see illustrations for definitions of the names of the different diamond facets ) ; "Crown" angles and "Girdle" thickness & symmetry are also important. Each of these proportion criteria have tolerance ranges which, however, are less crucial when violated than deviation from the above Depth/Diameter ratio ( "Percentage"). 
Marcel Tolkowsky is credited with calculating in 1919 the ideal proportions and facet angles that create maximum brilliance and fire. Unfortunately the "ideal cut" results in smaller weight yield from the rough diamond crystal and is rarely practiced. Most cutters today slightly compromise Tolkowsky's "ideal cut" but still create impressive results. 
When the diamond is well proportioned the path of a beam of light is returned directly back to the eye instead of escaping through the bottom or sides of the cut diamond, as a result the diamond will be more lively and brilliant. 
The Gemological Institute of America (GIA) scale for the diamond's Cut (proportions) is described using the terms "Excellent", "Very Good", "Good", "Fair", and "Poor" ("Recut").  
Clarity refers to the inclusions and blemishes in the crystal. Gemologists refer to these blemishes, rather, as identifying characteristics avoiding any negative associations and connotations. One must remember that a diamond is a natural substance and any inclusion or pattern of inclusions can be considered as the diamonds unique natural characteristic and "fingerprint". They can also help identify the diamond making your diamond as unique as a snowflake, since no two are exactly alike. 
Diamonds are graded for clarity according to the number, size, location and type of inclusion. Obviously, less numerous and smaller inclusions that are less centrally located are more desirable than the opposite. Examples of the type of internal inclusions
include: "pinpoints", included crystals that are transparent, opaque or carbon . Groups of pinpoints are called "clouds" and fractures are called "feathers". External blemishes include polishing lines, grain lines ( mineral growth twinning planes), scratches,
chips, nicks and naturals (part of the rough diamonds original surface or "skin"). 
Diamond Clarity Grading Terminology :
As the value of a diamond is determined, partially, by its "purity" or "clarity", standards for measuring the diamond's clarity have been put into place . The clearer the diamond is the rarer and therefore, the more valuable it is . The following terminology is used by the industry. The letter abbreviations are sometimes confusing to the novice but still are of high importance . 
The G.I.A Clarity Scale:
Flawless diamonds in this category are free from internal and external imperfections when examined by skilled professionals under natural or artificial light with a 10X loupe corrected for chromatic and spherical aberration or with a 10X binocular microscope equipped with dark field illumination. ( The following conditions still qualify a diamond as flawless: an extra facet located on the pavilion which cannot be seen from the face-up position; a natural that does not extend beyond the maximum width of the girdle or break the symmetry of the girdle outline and is not seen from the face-up position ) 
Internally Flawless diamonds in this category are free from all internal imperfections (inclusions) but do possess minor surface blemishes. Normally these diamonds may be made flawless by minor repolishing with the exception of surface grain lines. 
VVS1 and VVS2  
(Very Very Small Inclusions, level 1 or 2)  
These grades contain minute inclusions so small or insignificant that they are difficult to locate under 10X loop. When these inclusions are very difficult to locate visible only from the pavilion side or tiny enough to be easily removed by repolishing-the  
first VVS grade applies. Pinpoints, faint clouds, tiny feathers or bruises characterize the VVS grades.
VS1 and VS2  (Very Small Inclusion, level 1 or 2)  
These grades imply minor inclusions of a size, number and location that stand between those somewhat difficult to observe and those somewhat easier to observe under 10X loop. Small included crystals, small feathers, distinct clouds characterize the VS grades. 
SI1 and SI2 (Small Inclusions levels 1,2)  
These grades contain noticeable inclusions which are easily visible under 10X loop. Normally these inclusions will be centrally located and noticed immediately when the diamond is examined. diamonds in these grades (particularly the SI2 grade) may  
disclose inclusions to the unaided eye when placed table down on a white background, but not when viewed face-up.
I1, I2 and I3 (observable Inclusions levels 1,2 & 3)  
The "Imperfect Categories" contain obvious inclusions when viewed under 10X loop and are visible to the unaided eye in the face-up position. The grades vary from diamonds with inclusions that are difficult to locate by the unaided eye to those with  
severe inclusions that are easily noticeable.
Though a diamond is said to be "flawless" if no inclusions (internal or external) can be seen under 10 power magnification (triplet loupe or binocular microscope) truly flawless diamonds are extremely rare.  
The VS1 category is the cut-off point for what is considered to be a fine clarity grade since the inclusions characteristic of this grade are extremely small and difficult to see at 10X magnification. To the left of the scale from the VS1 grade the differences between each clarity grade are significantly smaller than clarity grades to the right of the VS1 category, where the tolerances for each grade are larger. SI grades are still considered to be very good to good, since their inclusions are not eye visible, especially 
when examined under normal lighting conditions. Under gemological laboratory conditions where daylight is used the inclusions in the SI2 grade are eye visible when viewed through the back of the diamond (known as the "Pavilion View"). SI grades are considered good choices especially when combined with good color and cut since the cost due to the clarity is usually within popular price ranges and budgets.  

Paradoxically enough, the more colorless the diamond, the greater is its rarity and value. Although many diamonds may appear colorless to the untrained eye, the majority contain very slight traces of yellow or brown. Color quality is critical because a diamond's value increases dramatically the more colorless it is. Color determination consists of comparison with Master-Color diamonds under daylight conditions. A single increase in a diamonds color grade can boost a diamond's value by thousands of dollars depending on the size and clarity grade combinations. diamonds colors are graded from the letter "D" (Colorless) to "Z" (prominent hue).  

Diamonds are weighed on a carat scale. Trade legend has it that the weight unit's name, "Carat", is derived from the measuring unit of antiquity: the carob fruit seed... There are 100 points in one carat. Therefore, weight can also be understood as a ratio (i.e. points/ 100). A fifty point diamond -half a carat- is then 50/100's of a carat. The size of a diamond measured in millimeters can also assist in determining approximately the diamond's carat weight. by using the following formula (not for the novice...):  
[Average Diameter in mm. ] X [ Depth in mm. ] X 0.0061 (whereby "X" stands for the mathematical operator "times" and 0.0061 is the Size/Weight conversion factor). It is however important to understand that there is no direct relationship between size and weight (i.e. a fifty point diamond is NOT twice as large (or wide) as a twenty five point diamond, only twice as heavy).  It is also important to note that size alone is not enough to determine a diamond's value : One has to consider also the cut and proportions of the diamond, its clarity as well as its color. A large diamond holds little value if it lacks brilliance, purity and good grade color.