Hi Peach,
For the engagement solitare ring, diamonds are valued according to their 4 Cs. (Color, cut, clarity, carat)
Color: D is colorless (best among "white" diamonds), E, F, G...Z (Worst among the "white diamonds, most yellowish under sunlight). *Pls note that for colors beyond Z is different, the diamonds are no longer considered as white diamonds, they can be pink, yellow...etc and their cost is much more than white diamonds)
Clarity: I can't really rem the different grades of the clarity, think the clarity is based on 3x scope that is used by most jeweller.
There is the internal flawless (not totally flawless, but flaws can't be seen in 3x scope), VVS1, VVS2, VS1, VS2 ...I(can't rem the grade, but the worst means can be seen with naked eye.)
Cut: This determines the brightness of the diamond. There are different theories for the ideal cut. E.g Larry, Hearts and Arrows in Goldheart have their own theories of what cut will make the diamonds more brillant. There is the brillant round cut (i tink). Dependant on the table to the crown ratio... This u will have to do more read up on.
Carat: size of the diamond. In my personal view, not impt. Average is 0.3cat and above for engagement ring...