A diamond certificate is the evaluation by a third-party, not by either the diamond buyer or seller. Unfortunately, an 3rd party certification is necessary, as it would be easy for an unscrupulous jeweler to take advantage of an uninformed buyer and sell him or her a stone, which ostensibly has much better characteristics than it really does. There are a number of ways unsuspecting buyers might land a bad deal. 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 GS Labs, 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. Internal fractures, inclusions, and excessively yellowish color, can be treated to visually improve the appearance of the stone. Such treatment deteriorates the overall quality and value of the stone. For example, diamond fractures can be filled with molten glass (a process known as Yehuda, named after the inventor’s name) to improve clarity by one or two grades. Or it can be worse: instead of molten glass, some jewelers use cheaper materials containing bromine to fill the fractures, which inevitably make the diamond darker over time, due to ultraviolet exposure. Some diamonds can be laser-treated to visually eliminate some of the “inclusions.” Laser treatment involves drilling tiny holes in the diamond to get to the inclusions. Needless to say, diamonds with drilled holes are worth less than comparable-grade diamonds that have no holes. Diamonds that are yellowish in color (color grades S through Z) can sometimes be pressure- and heat-treated to make them whiter (a process often referred to as HPHT, high-pressure and high-temperature). Such treatment also makes diamonds more fragile and brittle and, therefore, more prone to damage.
It is important to note that enhancement processes such as Yehuda, laser clarity enhancement, and HPHT can be useful for those who are looking to buy bigger diamonds for less money. If a diamond is inexpensive for its size due to yellowish color or if it contains a visible fracture, you can still buy such a diamond inexpensively (perhaps at a 30-50% discount compared to a naturally colorless or fractureless stone) and have it enhanced to improve its appearance. This enhancement would be significantly cheaper than buying a naturally higher grade diamond.
The problem, however, is that some jewelers make such enhancements and try to sell those diamonds without disclosing to buyers anything about the treatment.
To avoid buying a diamond “lemon” it is better to buy diamonds that are certified by reputable grading agencies.