Two diamonds with clearly different clarity's, have the same GIA clarity grade

It is often said in the diamond trade “I don’t buy paper show me the stone”.

In layman’s terms this means the experience diamond dealer will only buy a diamond that is graded by a laboratory with the expertise and equipment.

But as important as the laboratories report -- is that the dealer agrees with the grade given.

So what does it mean agree with the grade? After all it’s the GIA, correct?

Well not exactly:

The grade Si2 for example can be vastly different between laboratories like GIA and EGL for example, but we can also see great visual differences within the same laboratory. 

Why is this? 

And how can this happen?

Simple answer is the even GIA grading is done by different graders in different laboratories around the world. So does this mean GIA graders may be stricter or more experience than GIA graders in Mumbai? 

Simple answer is NO the grading system and the graders are all trained to the same standard and use the same equipment.

It can and does happen for a variety of reasons.

The Grader may be comparing stones while working, seeing similar stones with small variances over hours of working. Causing them to adjust their grading. 

The Grader could be tired, leading them to miss or not identify features.

Graders can use equipment incorrectly and get a different result. This can happen with a small adjustment in magnification or lighting for example.

So how does a laboratory normally get around this problem?

Easily, by using multiple graders to rule out subjective human error.

If three or more graders, grade the same stone then the average grade should be consistent for the same diamond every time. Not science but 100% effective.

So why are we looking at two differ stones from the GIA that are clearly a grade apart but have the same GIA grade on the reports. 

Good question I would like to know the answer myself. But then I also believe that “you don’t buy paper show me the stone”.