Here are a few ideas which can help work around the tricky problem of choosing a ring for someone without being able to ask them what they want.
Advice on choosing a ring
Option A: Ask a friend of theirs.
Maybe they know. Maybe not. Hopefully the friend can keep a secret too! But a close friend of hers can sometimes have an idea of what is more in line with her tastes.
Option B: Safe bet: Get a platinum solitaire presentation setting. Shape: round brilliant or princess cut.
This is one of the most popular choices due to the way they display the diamond beautifully. Therefore a solitaire presentation setting has the highest probability of being kept. Due to its simple straightforward design the diamond can easily be reset into a different ring later on if needed, or use the same gold/platinum to make a ring that your fiancee designs. Platinum has a nice sheen which emphasizes the brilliance of the diamond and does not distract from the diamond. Platinum is also the best choice in case of sensitive skin. Palladium is equally good. The most frequently chosen shapes are round brilliant, followed by a close second, the princess cut.
Option C: Notice what her tastes are. Does she usually like to wear gold or platinum. Does she choose more distinctive designs or prefer simplicity in the jewelry she wears. If she makes a comment about a TV or magazine advertisement, this is a good clue to her tastes.
Option D: Combine all of the ideas mentioned here.
The best surprise will be when you end up surpassing any expectations she may have had.