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Diamond Dictionary

 

 

The Diamond Dictionary

Culet
A culet is the point on the bottom of a diamond's pavilion. It is often faceted to remove the sharp point. Usually this facet is very small and nearly impossible to see. When searching for your diamond you should choose a diamond that has either no culet or a very small culet. Diamonds that possess a large culet are less desirable because the culet facet may be so large it may be visible to the naked eye through the table facet on the top. A large culet is common in diamonds that were cut in the early part of this century. They are often referred to as Old European or Old Mine Cuts. A large culet may also indicate that the original culet was chipped or broken off Below is GIA's and other gemological laboratories' list of abbreviations used on diamond certifications that describe the condition of a diamond's culet.

  • N or Non          No Culet
  • P                  Pointed
  • VS                Very Small
  • SM                Small
  • M                 Medium
  • L                  Large
  • VL                 Very Large
  • EL                 Extremely Large

Depth
The depth of a diamond is important to its brilliance and value. A diamond that is cut to Ideal or near Ideal depth percentages exhibits a balance of brilliance and is more valuable. Diamond cutters must remove more weight from the original rough diamond crystal to cut a diamond with proportions that produce great brilliance.

The facets on the bottom of a round brilliant cut diamond are called pavilion facets. When you look at a round brilliant cut diamond face-up the pavilion facets act as mirrors which reflect the image of the table--the large facet on the top of the diamond. This white table reflection seen within the center of the diamond indicates how beautiful and brilliant the diamond will be.

In a very fine to

Facet
The table below shows all the facets and their names on a round-cut diamond:

 





 

Fluorescence
Some gem-quality diamonds fluoresce: they emit light when exposed to long-wave ultraviolet light. How does this fact affect a diamond's appearance and value? In the past, some people in the diamond trade have considered moderate to strong fluorescence as a negative value factor for fine diamonds and a positive value factor for diamonds with a lower body color. Why? The trade perceives diamonds without fluorescence as "more pure" than diamonds with it. There is a perceived rarity for diamonds of fine color without fluorescence. Fine quality diamonds with strong fluorescence may be undervalued because rare extremely strongly fluorescent diamonds known as "overblues" have a visible haziness that makes them appear almost cloudy in light with strong ultraviolet content. Dealers have theorized that strong fluorescence may affect apparent clarity. At the same time, strongly fluorescent diamonds with a yellowish body color have long been considered to appear to have a better color because the blue of the fluorescence makes them appear more white in sunlight, which is a source of ultraviolet light. Lower-color diamonds with strong fluorescence sometimes command a premium. The Gemological Institute of America's Gem Trade Laboratory, the country's leading diamond grading lab, lists fluorescence as an identifying characteristic, not a grading factor. And based on a GIA GTL random sample of data for 26,000 diamonds, diamonds with fluorescence are more rare than nonfluorescent stones: 65 percent of diamonds have no reported fluorescence. Listed below is GIA's list of abbreviations for strength of fluorescence as well as their meanings:

  • N                No Fluorescence
  • F or FB       Faint or Faint Blue Fluorescence
  • SL              Slight Fluorescence
  • M                Medium Fluorescence
  • S                Strong Fluorescence
  • EF              Extreme Fluorescence

To help clear up some of the confusion around fluorescence, GIA commissioned a study on the impact of fluorescence on the appearance of diamonds and published the results in its professional gemology journal, Gems & Gemology in the Winter 1997 issue. The fluorescence study compared sets of round brilliant diamonds with a range of color grades in different controlled lighting conditions. The diamonds were evaluated by gemologists, dealers, and untrained observers. The results? Untrained observers could not distinguish any affects of fluorescence. Even trained observers did not consistently agree. In general, strongly blue fluorescent diamonds were judged to have a better color grade in the face up position. (No affect of fluorescence was observed in the table-down position, which is how diamonds are color graded by laboratories.) No relationship between fluorescence and transparency was apparent. The effect of fluorescence on color was most notable in grades I through K. The conclusion? "In the table up position (as is commonly encountered in jewelry), diamonds described as strongly or very strongly fluorescent were, on average, reported as having a better color appearance than less fluorescent stones. In this study, blue fluorescence was found to have even less effect on transparency. These observations confirm GIA GTL's experience grading millions of diamonds over the decades." The study seems to indicate that over the years, the trade has unfairly stigmatized diamonds with strong fluorescence. And these diamonds are often available at a discount because of trade perceptions of possible negative impact of fluorescence on the salability of stones. Because "overblues" are so rare, none of these diamonds were included in the GIA study. Overblues clearly do have a different appearance than ordinary strongly fluorescent diamonds. If you are ever in the Smithsonian institution in Washington D.C., you can see the most famous example of an overblue: the 127 carat Portuguese Diamond. While you are there, make sure to visit the famous Hope Diamond, which owes some of its legendary curse to the fact that it fluoresces an extremely unusual red, which is only known to happen in blue diamonds.

 

Girdle
The girdle is the outer edge of a diamond. The grade of a diamond's girdle is determined by the appearance of the girdle at its thinnest point and thickest point. A diamond's girdle can be faceted, polished smooth, or have a slightly granular appearance. Very fine cut diamonds often have faceted girdles. A diamond cutter must spend extra time to carefully facet a girdle's edge. A faceted girdle does not improve a diamonds grade. Most labs grade only the thickness of a diamonds girdle and not the surface appearance.

Ideal girdle thickness should range between Very Thin to Thick. Sometimes a diamond can have a perfect medium girdle around ninety-nine percent of its diameter and only be very thick at one very minute, isolated point. This diamond will receive a laboratory girdle grade as medium to very thick. Diamonds that have grades extremely thin, very thick or extremely thick are usually not recommended.

The Length-to-Width Ratios

Length-to-width ratios aren't the kind of thing most people think about when purchasing a diamond. However, this is an important factor that shouldn't be overlooked when buying diamonds of certain shapes. The length- to-width ratio is based on the proportions of the original rough crystal. Usually, the cutter is reluctant to sacrifice weight and value to meet the established ideal proportions for the chosen shape. Length-to-width ratio is only important in fancy cuts or diamonds that are not round. The length-to-width ratio, quite logically, is calculated by dividing the length by the width.

Polish
Polish influences how well light is able to pass through a diamond and is very important to a diamond's brilliance. It is therefore necessary to select a diamond which has a polish that is laboratory certified to be Good, Very Good or Excellent. Diamonds that have poor to extremely poor polish are less brilliant because they have microscopic polish lines that blur the surface of the diamond. These polish lines reduce the amount of light that enters or exits a diamond. When selecting your diamond, be aware many diamonds possess poor polish. Diamond cutters can greatly reduce labor costs by not taking time to properly polish a diamond.

The following are grades used to designate the quality of a gem's polish:

  • EX or E--Extremely Good   
  • VG or VGD--Very Good     Very difficult to locate under 10 X power
  • GD or G--Good     Difficult to see under 10 X Power
  • FR or F--Fair   Easy to see under 10 X Power
  • PR or P--Poor   Very easy to see under 10 X / difficult to unaided eye
  • VP--Very Poor   Relatively easy to see with the unaided eye
  • EX or EP--Extremely Poor    Obvious to see with unaided eye

Symmetry

Symmetry is a crucial element of a quality finished diamond. By symmetry here we mean the exactness of the shape and the balanced arrangement of the facets. To the unaided eye, finish features usually have little effect on appearance. Symmetry is less important in diamonds that have lower clarity grades. When selecting your diamond, make sure the grading report rates it as Excellent, Very Good or Good.

Table
The table is the large flat facet on the top of a diamond. The table has a direct effect on the sparkle of a diamond. The size of the table in concert with the angle of the crown is responsible for the balance between brilliance--the flashes of white light bouncing back to the eye from within the diamond--and the play of colors created by refracted light as it prisms its way through a diamond's facets. The following are the ideal table percentages for various shapes of diamond:

  • Emerald                       50% - 75%
  • Marquise                      50% - 62%
  • Oval                              50% - 62%
  • Pear                              50% - 62%
  • Princess                        65% - 80%
  • Radiant                          65% - 80%
  • Round                           52.4% - 57.5%

 

 

 

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