The Degree of Surprise
Shopping for and choosing the right engagement ring will be influenced by how much of a surprise your proposal will be.  If surprise is not an important element (for you or your partner) of the proposal, then in a way the process will be easier for you.  Some couples are even comfortable with having the ring recipient picking out her ring independently. 

If your proposal will be (or you want it to be) a surprise, then you will need to be stealthy and do a bit more work, but your reward in the end will be in knocking her spectacularly off her feet.

The Recipient’s Taste
This can be one of the hardest parts of choosing a ring.  What does she like?  If, as described above, receiving the ring will not be a surprise for her, you can directly ask her what she likes.  In this case, shopping for an engagement ring can be a really fun experience for a couple in which they can plan and celebrate their coming union.

(For an explanation of diamond grading, see Diamonds)

If you would like to surprise her, it is best to do some investigating to discover what will please her most.  Generally, women tend to have the strongest preferences for the color/kind of metal and the shape of the cut. (See gemstone information for examples of different cuts)  For example, she may know that she likes white gold more than yellow gold and square cuts more than pear cuts, but she might not know about grades of diamonds or have an exact size preference.  If it is possible to ask her without diminishing the experience for either of you, you can walk buy a jewelry window and quickly ask her “Hey look at these rings… hypothetically, which one do you think is best?”  You can also watch how she reacts to her friends or family’s engagement rings. 

(For an explanation of diamond grading, see Diamonds)

If these options don’t work well or if you’d like to be sneaky, you can audit her jewelry box.  If she has mostly silver and white gold, you can figure she does not want a yellow gold ring and vice versa.  See if she tends to like simple or complicated designs; one main center stone or lots of small stones, if there are any persistent “favorite” colors of stones.  It is important to exclude jewelry pieces you know she didn’t choose herself or hardly ever wears.  You could even try to borrow her jewelry for a day to take it to a good jeweler (usually small independent shops are best) and ask, “This is what she usually likes to wear.  What kind of engagement ring do you think suits her style?”   

Another way to find out what she likes without arousing her suspicion is to enlist the help of a female friend or family member.  Discussing engagement rings is a perfectly normal conversation among females, so if her sister or friend casually brings it up (and then passes on what they learn to you), she probably won’t suspect anything.

(For an explanation of diamond grading, see Diamonds)

The Recipient’s Expectations:
Another tricky aspect of choosing an engagement ring is the recipient’s expectations.  This is a separate category from what she “likes” because engagement rings are socially and culturally symbolic.  For example, her favorite stone may be blue sapphire but she might (even subconsciously) expect a diamond because that is the norm of her culture.  Generally in the USA, diamond is the expected stone for an engagement ring, so unless your lady is a trendsetter, unconventional, “likes to be different” or has expressed openness to non-diamond engagement rings, go with a diamond.  (However if she does belong to one of the previous descriptions, seriously consider another gemstone).

Expectations also play a role socially in that, well, if her mom and sister and all her friends have big flashy rings, she will probably be disappointed with a small one (unless she has expressly stated “Uggh, I hate those ugly big rings!”).  Even the sweetest ladies subliminally enjoy to one-up each other.  So if you can make sure her engagement ring is even a little bit better in some way than those of her female peers, you will give her the chance for some delicious social gratification. 

If you know what she would like, it is still important to make sure the engagement ring reflects your own personality and that the engagement ring contains a special story or meaning.