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How Good Is Your Diamond? GIA Might Have An Answer.
Ever wonder why it is that some diamonds just sparkle and scintillate like heck, while others look dull, glassy, and lifeless?
The answer is it has everything to do with the Cut of your diamond. The better the Cut, facet alignment and proportion, the more light will be coming out through the top of the diamond to your eye.
GIA (Gemological Institute of America) the foremost diamond grading and research laboratory in the world is debuting its new Cut Grade for round brilliant shape diamonds on January 1, 2006. Based on extensive research over fifteen years the new Cut Grade will appear on it's Lab Grading report. Cut Grades will range from Excellent to Poor.
In it's research, GIA considered, evaluated, and measured all 58 facets that compromise the round brilliant shape.
Facets that comprise the round brilliant diamond.
The grading system takes into account the entire diamond including the Table, Crown, Pavillion, Stars, upper and lower girdle facets, and culet size.
Cut Grade will range from Excellent to Poor and Girdle Thickness will factor into determining the final cut grade. In order for a diamond to get an "Excellent", girdle thickness will have to be in the range of 'thin' to 'slightly thick'.
Cut Grade Designations.
Girdle thickness will play an important consideration in assessing the final Cut Grade. This is so because many times in order to reach a target carat weight, a cutter will leave a thicker girdle in order to retain more weight. This will necessarily come at the expense of light performance and diamond beauty.
Evaluation of Final GIA Cut Grade based on Girdle Thickness.
The Cut Grade is based on rounded averaged values in the folllowing way:
Total depth is rounded to the nearest 0.1%
Table size is rounded to the nearest 1%
Crown angle is rounded to the nearest 0.5°
Pavilion angle is rounded to the nearest 0.2°
Pavilion angles ending in odd numbers are rounded UP to the next even number.
Star length is rounded to the nearest multiple of 5%
The lower-half (lower-girdle facet) length is rounded to the nearest
5%.
If you're considering a diamond that is not GIA graded you will need either a Megascope or Sarin Analysis Report which will provide you with the measurements for the Star, upper and lower girdle facets, which you can then enter into the GIA Facetware calculator along with the other numbers to determine the Cut grade.
The broad range in numbers rounding by GIA was based on the variances they found in measuring devices currently on the market and the observable differences in cut quality that could be distinguished by laypeople and tradesmen in the course of their research. No doubt this broad range may very well take an "Excellent" into a "Very Good" despite the certainty that a visual difference will not be discernable. What effect this will have on prices remains to be seen.
Attached are illustrations of the different facets in a round brilliant diamond used by GIA to calculate the Cut Grade as well as their Facetware interface and Cut Grade definitions. Also included is a Megascope Cut Analysis with the Star facet values and the lower girdle facet values highlighted.
GIA Facetware Calculator to obtain Your Final Cut Grade. If you're considering a diamond that has not been graded by the GIA, you will need a Sarin or Megascope Cut Report for some of the requested measurements.
Below is am example of a Megascope Cut Analysis report providing the measurements to plug into the GIA Facetware calculator.
Consumers will definitely benefit from this new Cut Grade system.
Comments
Can you tell me how to find table size of a diamond in detail
Posted by: Jeyanthi Sriram on October 10, 2007 5:39 AM