A diamond certificate is also known as a Diamond Grading Report. This report comes from the Gemological Institute of America (GIA), and you should require this report when you are purchasing a diamond.
With a diamond certificate, you can verify the color, cut, carat, weight, and clarity of the diamond. You don't have to worry about a diamond dealer telling you anything less than the truth, because the certificate comes from the GIA not the dealer. You may be required to pay for the certificate, but the cost is usually low, and in many cases, it will help you negotiate a better price on the diamond or keep you from purchasing a lower quality diamond altogether.
If you buy a high quality diamond, and then later decide to sell the diamond, you will need to have the certificate, or you will have a hard time selling it to someone else. Furthermore, you can use the Diamond Grading Report to look up the wholesale value of the diamond in question. Use the guide that is used by the diamond cutting industry.
With the Certificate, or Diamond Grading Report, there won't be any doubts when you are trying to purchase a diamond. You can easily find out what the diamond is worth. This will prevent you from overpaying, and it can prevent a seller from under-charging as well.
A copy of the Diamond Grading Report should be given to your insurance company as well, when you insure the diamond. This provides absolute, unquestionable proof of the value of the diamond should it be stolen in the future. Insurance companies cannot argue with the report.
Avoid diamond dealers who seem reluctant to provide a certificate! Also avoid sellers who tell you that a certificate diamond will cost you more the only additional cost should be the cost of the certificate, which is low. If the dealer doesn't want to provide a certificate, then you don't want to do business with that dealer.
Don't accept certificates from Gemological Laboratories other than GIA. There are many fly-by-night Gemological labs these days, but in the end, GIA has been established as the most respectable and trustworthy not to mention oldest of the lot. So avoid dealers who don't want to use GIA for certification purposes as well.
Don't buy an expensive diamond without paying the extra cost of the certificate. If a dealer tries to convince you to make the purchase without the certificate, or if they want to use a company other than GIA, you can be sure that the dealer has probably greatly inflated the price of the diamond they have something that they are hiding from you.