The Basics
Buying an engagement ring can be broken down into two steps: (1) choose a rock, and (2) choose a setting. You can buy the rock and the ring together or separately, but you should think of them as independent decisions.
The Rock
First, pick what type of stone you want (diamonds being the most common). Then, pick what quality of stone you want, by evaluating the 4Cs: Cut, Color, Clarity, and Carat.
The Setting
When it comes to settings, there are again two factors to consider. First, setting type, i.e. the style or overall design of the setting. Second, which type of metal to use for the setting.