First, determine your budget. One general rule of thumb when buying a diamond engagement ring is "two months salary." This is just a guideline, it's not carved in stone and your first consideration should be what you can comfortably afford.
Deciding on carat size is really about striking a balance between size and quality. If she prefers larger jewelry items, and you are working within a budget, you can still find a larger diamond of excellent quality by selecting one which is graded slightly lower in terms of color and clarity.
Remember that slender fingers make small diamonds look bigger. If she has small fingers, a 1-carat diamond will look proportionately large -- and an even larger stone may appear stunningly big!
Think about what sort of setting will hold the diamond. You'll have to be sure that the setting you choose is made to fit the carat weight of your diamond.
Learn about the 4 C's of Diamonds before you begin shopping.
Loose Diamonds and Grading Reports
A grading report is a "blueprint" of a diamond, it tells you the diamond's exact measurements and weight, as well as the details of its cut and quality. It precisely points out all the individual characteristics of the stone. Grading Reports also serve as proof of the diamond's identity and value. A certificate is not the same thing as an appraisal.
A grading report describes the quality of a diamond, but it does not place a monetary value on the gem. An appraisal places a monetary value on your diamond, but does not certify the quality of the diamond.
There are many diamond labs that issue grading reports, but the Gemological Institute of America (GIA) and the American Gem Society Lab (AGSL) are the two most widely regarded and recognized diamond grading labs in the world.
Shopping for diamonds with grading reports allows you to make an informed choice about your selections, and to comparison shop. You can compare one diamond with a particular weight and quality with other diamonds of similar weight and quality to determine which is the better value.
If a jewelry store offers to sell you a loose diamond without a grading reports, keep in mind that it means you are buying the diamond based only on the salesperson's claim about its quality, and that a trained gemologist or even other jewelers may disagree with the salesperson's assessment.