Better quality diamonds weighing a half-carat or more will generally have a certificate, and there are only a few gem labs to grade all these stones. Who is assigning the color and clarity grades to these hundreds of thousands of diamonds? Many of those graders are newly graduated gemologists. Working as a grader in a gem lab for a few years will look very good on a gemologists resume, but it is a stressful job, and you can be sure mistakes are made. When a diamond is graded better than it really is several things can occur. The jeweler or Internet source can be greedy, and make a larger profit. Or, the jeweler or Internet company can sell it based on a normal markup, and it will make their diamond appear to be a bargain when compared with a correctly graded diamond. When a diamond is graded poorer than it really is, the gem lab is always asked to reconsider the grading, and many times the lab will change the grading to the jewelers liking. If the lab doesn't change the grade, the jeweler may send the diamond to a different gem lab and sell the diamond with that lab's certificate instead.
In some respects the consumer was better off before certificates became the norm, because the grading of diamonds has been taken out of the hands of the experienced local jeweler. Once a diamond has a certificate most jewelers or Internet companies are only guaranteeing the diamond is the same diamond being described in the "certificate," and not that the grading is correct. The consumer has dictated to the jewelry industry that the center diamond in a piece of jewelry should have a certificate without realizing there would be good certificates and bad certificates. Of course no one wants to buy a diamond with a bad certificate, but how do you know you're buying a diamond with a good certificate?
While I'm certainly not suggesting that most certificates are wrong, I am saying that if you are about to spend thousands of dollars on a diamond, from someone you've never dealt with before, my opinion of the diamond should be very important to you. Just remember the motto, "buyer beware," and realize a certificate is only a piece of paper.
Most likely this is going to be the most expensive jewelry purchase you will make for quite some time, so you will want to be sure you're getting exactly what you think you are paying for.