Should a diamond come with some kind of certification?

Usually it's for the benefit of an insurance company so they know how much to pay out in the event of a loss.

Anyone can claim they own a carat diamond ring, but it's the certificate that verifies proof of actual ownership

Makes sense. I've also been reading that you need a grading report to verify your diamond's stats...four Cs, proportions, etc. I've heard of a few labs, but GIA seems to be the most reputable. Would you say that's true?

diamonds should come with an explanation of why they're worth the money your paying for them

I guess I don't see why this is so important. I mean, isn't the question at the end of the day: do you like the way it looks?

With diamonds I never worry about the resale value, because anything that you buy, walk out the store, and see an instant 30-50% drop in value, isn't an investment or something you should consider reselling in the future.

Cycloptichorn

I don't care about the resale value. I care more about being able to get it insured at the right value and being sure I got what I paid for and that it hasn't been artificially enhanced in any way. That's why I'd want it to be graded as accurately as possible by a reputable lab. That's my basic understanding of why you get a grading report.

I would not buy a diamond that is expensive unless it was certified because you run the risk of paying way too much for the diamond. Then I would get an independent appraisal for insurance purposes. Robert Shipley and others started AGS and he founded GIA.

TTH: This is good idea never buy anything online without certification. and diamonds need GIA certification for diamonds quality.

If the diamond is not certified, there is a good chance you may be buying a stone that is one or more grades below stated values in terms of carat weight, color, clarity, or cut. Without a certificate, issued by a reputable agency, such as GIA, AGS, or EGL, or HRD, a buyer relies on a jeweler’s integrity that the diamond’s 4C parameters are accurate and not overstated.

A lack of a certificate may be a clue that the diamond has been “enhanced” and the jeweler knows an inspection would reveal the diamond’s true condition, as well as indicate how the compromised diamond might erode.

Several well-known grading agencies in the United States and Europe have high reputations:
Gemological Institute of America (GIA)
American Gemological Society (AGS)
European Gemological Laboratory (EGL)
Diamond High Council (HRD, which is an abbreviation of the Dutch “Hoge Raad voor Diamant”)
International Gemological Institute (IGI)
International Confederation of Jewelry, Silverware, Diamonds, Pearls, and Stones (CIBJO)