GIA Grading Proceedure

[Krackr]

Can someone confirm when GIA grades a diamond it is done by 5 gemologists with the highest and lowest score eliminated and then average the remaining.I read reference to this but can't remember where.

 

[denverappraiser]

Grading is divided into different steps. Weight, measurements, clarity, color and so on are all done by different people. The procedure varies slightly depending on the stone but clarity, for example, is graded twice by two separate graders in isolation. If they agree, that’s the grade. If they disagree it gets sent to a third as an arbiter. Measurements are only done once unless there's some reason to believe an error has happened. Weighing gets done dozens of times although the ‘official’ weight only gets recorded once and the rest are simply checks. Presumably a discrepancy would result in some fur flying. The total number of people who work on a stone is way more than 5, but it takes a pretty unusual scenario before 5 separate graders consider at a particular attribute.

 

[Diamond_Hawk]

From gia4cs.gia.edu: (emphasis added)

Grading Diamond Color

Since light source and background can have a significant impact on a diamond's appearance, diamond color is graded in a standardized viewing environment against color masters. A minimum of two color graders enter their independent evaluations into the system and depending on the agreement of these grades, and the weight and quality of the diamond, it may be sent to additional graders who enter their own color opinions. The grade is not determined until there is sufficient consensus.

Grading Diamond Clarity

Diamond clarity is graded under standard viewing conditions with 10x magnification. The preliminary grader carefully examines the diamond in order to identify clarity/finish characteristics and evidence of any clarity treatments such as fracture filling or laser drilling.

A minimum of two graders assigns their impression of the diamond's clarity, polish, and symmetry. Next they plot the clarity characteristics on the diagram most representative of the diamond's shape and faceting style.

Grading Diamond Cut

GIA provides a diamond cut quality grade for standard round brilliant diamonds that fall into the D-to-Z color range. To develop their Cut Grading System, GIA performed extensive computer modeling of round brilliant diamonds over a 15-year period and conducted more than 70,000 observations on actual stones to validate the research. This system can now predict the cut grade for more than 38.5 million proportion sets.

GIA's Excellent to Poor Cut Diamond Grading System assesses the diamond's overall face-up appearance to predict the intensity levels of brightness, fire, and scintillation (the diamond's sparkle and interplay with light). GIA also screens every diamond submitted to determine if it is synthetic.

Diamond Carat Weight Measurement

To determine diamond carat weight, the diamond is weighed using an extremely accurate electronic micro-balance that captures the weight to the precise fifth decimal place (the nearest ten-thousandth of a carat). An optical measuring device is used to determine the diamond's proportions, measurements, and facet angles.

 

[Krackr]

DA.....Thx for the detailed reply I feel much better informed now .

 

[Texas Leaguer]

The labs also have protocols that provide for more graders and/or more senior graders taking part as the stone sizes/values increase.
So a large stone will be seen by more graders with more experience than normal goods running through the pipeline.