The 3C's  Color, Cut and Clarity... all of which determine the 4th C... Cost.

Color: Diamonds are rated from D (whitest) to Z (Brown). The closer you are to D the higher the cost. An engagement Diamond is typically between between D and G in Color.

Cut: The most fascinating cut is the Round Brilliant!....

Clarity: Diamonds are rated as to how pure they are and how many inclusions (flaws) they may have.

F= Flawless or IF= Internally Flawless

VVS= Very Very Slightly Included

VS = Very Slightly Included (Also VS1 and VS2 are used)

SI = Slightly Included

I = Included (Typically avoided)

Typically, the minimum acceptable as a wedding diamond (unless you can afford more), would be SI or VS2.

Cost: A general order of magnitude of an average engagement ring is $3,000.