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At Blue Nile, our certified diamonds have been graded by either the GIA or the AGSL. These diamond grading laboratories are two of most respected and prestigious in the world. They both approach the science of cut grading in very similar ways, so making diamond comparisons is simple even if the grading reports are from different labs.
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GIA bases their new (as of Jan. 2006) cut grade on a combination of face-up appearance, design and craftsmanship elements that all contribute to the diamond's fire and brilliance. They employ a predictive computer model,
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based on over 70,000 individual diamond observations and 38.5 million proportion sets, to determine a diamond's brilliance based on its interrelated measurements. Most GIA diamonds graded prior to January 1st, 2006 will not have a laboratory-assigned cut grade. The GIA cut grade system includes ratings of Excellent, Very Good, Good, Fair and Poor. GIA does not assign an Ideal cut and an Excellent rating is their highest grade. For comparison purposes, a GIA Excellent cut will be listed as a Blue Nile Ideal cut on our Web site. The cut grades are equivalent. Learn more about the GIA Diamond Grading Report and Diamond Dossier®.
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The AGSL determines cut grade using light performance, proportion, and finish characteristics to determine a diamond's fire and brilliance. When the AGSL determines a diamond's cut grade, they look at a combination of 11 different criteria in these three categories,
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and then assign cut grades of Ideal, Excellent, Very Good, Good, Fair, and Poor. The AGSL labels a diamond that meets their standard of perfect proportions, polish, and symmetry as "Cut Grade: Ideal". An AGSL Ideal is known throughout the diamond industry as a "triple-zero" grade. An AGSL Ideal cut is the equivalent of a GIA cut grade of Excellent or a Blue Nile Ideal cut. For comparison purposes, these cuts are all listed as Ideal on our Web site. Learn more about the AGSL Diamond Quality Document.
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If a GIA or AGSL certified cut grade is not available for a particular diamond, Blue Nile's proportional grading system will determine the diamond's cut.
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Blue Nile Cut Standards
Based on criteria listed on both grading reports:
Table percentage
Depth percentage
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Blue Nile uses the measurements from each diamond's grading report to determine:
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- Table percentage: The ratio of the table diameter of the diamond to its overall diameter.
- Depth percentage: The ratio of the depth of the diamond to its diameter.
From these proportions, Blue Nile assigns each diamond a cut grade. Here are the proportion ranges for each grade of diamond cut:
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Cut Grade
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Depth %
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Table %
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Signature Ideal
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60.1 - 61.9
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55 - 57
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Ideal
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60.1 - 62.8
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53 - 57
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Very Good
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58.5 - 64
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52 - 60
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Good
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57.5 - 64.5
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51 - 65
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Fair
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56.5 - 67
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50 - 67
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Poor
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<56.5 or >67
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<50 or >67
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