As far as I know it may have originated with eggs (grin)!
Seriously, I have never run across a standard for this in my GIA
course or anywhere else. I think these grades would mean
whatever you (or whoever) decided to make them mean. They are
ordinal rather than ratio scaling, meaning that AAA is better
than AA is better than A and that’s about all you can say for
sure. In other words, AAA could be $1/ct or $200/ct and then AA
could be from $199/ct down to $150/ct or down to $30/ct. The
absolute value of the grade is not specified, nor is the "width"
of the category either. We had a discussion about color grading a
while back on Facetor’s Digest and no one came up with a really
good standard except for the GIA Gemset or the GemDialog system
and both of those are fairly costly. HTH.