Okay... just saw a TV ad for one of the smaller nationwide jewelry retailers and it makes me want to scream. They are advertising "Certified" diamonds for a "great price". Don't fall for this!
"Certified" means that some diamond lab has graded the diamond. This means they have given their opinion about the color, cut, clarity, and carat weight of the stone. "Certified" does not mean that the diamond is a good diamond - it just means that it has a piece of paper with someone's opinion on it.
There are dozens of diamond labs that certify diamonds and here's what you need to know - some diamond labs are misrepresenting the quality of the diamond. GIA (Gemological Institute of America) is the most reputable grading lab - they set the benchmark for all other labs. EGL USA provides a reputable diamond certification - they tend to be slightly more "forgiving" with their certifications, but generally provide a fair representation of the quality of the diamond. Beware of the diamond grades from any other diamond lab. I cannot tell you that they will be wrong in every case, but some exist so that jewelers can represent the diamond as something other than it really is. This means that the "certification" says one thing, but the diamond is something else... and you paid for what the paper said.
In the case of the "2 Carat Certified Diamond" that gives you "factory direct savings" for under $5,000 that I saw in the TV ad... I did some digging and it is a 2.09 Carat; I2 Clarity; L Color; Very Good Cut. What this means is that it is significantly flawed in terms of inclusions - you will be able to easily see large dark spots and other "marks" inside the diamond with the naked eye. It also means that it is very yellow in terms of color. There is not enough other information provided to determine just how the cut of the diamond detracts from its sparkle and brilliance. They also don't tell you which lab has certified the diamonds so it is highly likely that the diamond is actually worse than described.
I searched 2 reputable online diamond sites and they don't even list or try to sell any diamonds that are this bad so it's impossible for the consumer to understand what they are really buying, how it compares to other diamonds, or the real value of what they are buying.
I actually have no issues with someone purchasing a diamond based purely on size. What I do have a big issue with is how some jewelers represent that diamond to an uneducated buyer.
The point here is that don't be fooled by ads like this. Choose a jeweler that you can trust, will work with you to put together the best ring at a price you can afford and will stand behind everything they sell you. If you really want a 2.09 Carat; I2; L diamond, your jeweler will be able to find one for you at a comparable price to the "factory direct sale price" in the TV ad.