I am not familiar specifically with IGI's colored stone certificates.
But, here are some guidelines to follow when you look at any lab
cert. Each lab has its methodology, prejudices, strengths and
weaknesses. This is simply because the trade has managed to keep the
principles of quality grading gemstones a secret even from
themselves. Lab gemologists come from different traditions with
differing approaches and opinions. Dealers are well aware of this and
tend to exploit it, that is, they will send a stone to a lab that is
likely to give the best cert to a particular stone. So if you are
dealing with, say a padparadscha sapphire, labs have different
definitions and the dealer will send the stone to the lab whose
definition his stone fits.
Votaire said: If you would speak with me, define your terms. Until
recently labs did not even use a standard terminology and in the
trade dealers use various terms some of which are simply inaccurate.
There is beginning to be a generally accepted terminology for
describing color. GIA's new colored stone course does follow that
terminology and by doing so has moved the standardization process
There is still no generally accepted explicite colored stone grading
system although I contend there is certainly an implicte one that
exists in the marketplace. I have tried to make the princples of this
system explicite in my book.
For Information and sample chapters from my new book: