Never buy a diamond just because it comes with a certificate. A certificate isn't enough. You need to check the details on the certificate and understand the report . You need to know the 4 Cs. Also, get a certificate from a reliable laboratory such as Gemological Institute of America (GIA). Certain laboratories may even provide a value of the diamond . Be aware this is just an estimated or inflated value. So if you want to insure your diamond make sure you get it appraised.

It is always better to buy a loose stone. Since the bulk of a diamond ring comes from the diamond itself , it will wiser to buy a separate diamond. By buying a complete diamond ring you won't know the breakdown of the cost and the jeweler can set an arbitrary price. Also , you can examine the diamond in its entirety as the setting would hide part of the diamond and perhaps its flaws. Also, a diamond ring may come with a setting you may not like. You can specify a specific setting if you buy a loose diamond.

Find out whether the diamond you buy is treated. Clarity in the diamond may be improved by filling up the cracks with molten glass. Laser can be used to burn a hole in the diamond to reach black blemishes . The color of a diamond may be also be improved or changed by high pressure-high temperature annealing and irradiation respectively. Ask the jeweler if the diamond has been treated. If it is not, then request for a certification of this fact. If you are on a budget and intent to buy treated diamonds , find out whether the jeweler provide any warrantee for the treated diamond.

Don't simply buy a diamond because of its weight . You need to consider other aspects as well. Which do you think is better ? A bigger but poorer quality diamond or a smaller but better quality diamond ?A lighter stone with a better cut is worth more than a heavier but undercut stone. A diamond may weigh heavier because it may be cut to maximize its weight and size but it is not cut to its ideal proportion that would bring out its fire and brilliance

Also don't be misled into thinking the more facets a diamond has the better it is . Nowadays you can find diamonds with over 70 facets being promoted as being better than 57 facet diamonds but this is not true. It is the diamond cut which is the most important not the number of facets. The diamond facets must be cut with proportion and perfect symmetry. The diamond must be cut in such a way that light is refracted out through the top of the table which gives it the sparkle and brilliance and not through the pavilion and bottom. Many diamond shapes have less facets than the brilliant round diamond but yet are as beautiful as the round diamond such as emerald and princess cut diamond.