GIA does not just look up which Argyle grade corresponds to which GIA grade.
It doesn't work that way.
There is no such Argyle-GIA cross reference chart.
GIA plays second fiddle to no one.
GIA grades the FCD using their lighting, their standards and their reference stones.
I try to read lots about FCDs and their grading and I've never read that GIA pays any attention to color grades assigned by Argyle, or by anyone else.
If two browns, Argyle graded C4, are submitted GIA may grade them different color grades.
My observations is FCD color grades from GIA hold more weight in the marketplace, and in pricing, than color grades from Argyle.
The skeptic might even wonder whether Argyle, like Tiffany, has a conflict of interest when 'grading' their own stones, especially now that most of the diamonds that Argyle can economically mine have been mined.
The FCDs Argyle is mining today are declining in number and quality.
I just reviewed Argyle's 2015 Tender and the stones are smaller and more-included than ever.
Many would have NEVER made the Tender bar 15 years ago.