They can be appraised, but I don't think a GIA certified diamond can be re-certified, or even worse, decertified. Unless the gemologist was blind in one eye and couldn't see out the other, GIA certifications are good for the life of the diamond.

So moral is, never use some company's in house certification specialists. They can make stuff up, but GIA certification overrides all others.

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman