Since i have done a good amount of research in the past few weeks just going to add the following to those who come after me.
When looking for a diamond, there are certain measurements which you want your diamond to be in. This will help weed out the bad ones that don't perform as well.
For Round brilliants:
depth - 60 - 62.5%
table - 54- 57%
crown angle - 34- 35 degrees
pavilion angle - 40.6- 41 degrees
girdle - avoid extremes, look for thin to slightly thick, thin to medium etc
polish and symmetry - very good and above
You need to also make sure that the certification is either GIA or AGS. EGL is being banned by many trading networks as of late and tends to grade diamonds higher than they really are (Hence the reason they are cheaper).
Also, if you can, ask the vendor for an
idealscope or
ASET image. Basically, these measure the light performance of a diamond. Its easier to tell which diamond is better using these than with the naked eye(at least for me) since most jewelry stores lighting will make all stones look good. I know i really couldnt tell the difference in many cases.
Once you have a good cut, then you can decide what size and color based on your budget. I would probably stay H or better and SI1 or better. But you could get a good deal if you find a I color SI2 that is superbly cut thats eye clean or has an inclusion that could be hidden by a setting.
Good luck