GIA vs AGS certs

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What is the main difference beteen these two types of certification?

Does certain diamonds only 'qualify' for either cert?

 

[Garry H (Cut Nut)]

There is little if any difference in color / clarity and none in weight exceept AGS list to .001ct where as GIA round down.
AGS 1.008ct = GIA 1.00ct AGS 1.000ct = GIA 1.00ct
AGS 1.009ct = GIA 1.01ct

But AGS give more cut info - but the info means they sell for more because in theory only the best cuts go to AGS. In practice many if not most AGS 0's (top cut grade) are inferior because they use a system based on a 1919 theory (Tokkowsky) that was wrong. see http://www.folds.net/diamond/index.html

So I buy and sell GIA but make sure they look good first and save some money. You can too with HCA and an www.ideal-scope.com if you are prepared to put in some effort.