An engagement ring should be worn by a lady for the rest of her life, she though isn’t the one to choose it. Some fiancés to be may have a good idea that their partner is about to pop the question and may choose to give hints and point out rings in shop windows, if the man is planning a surprise though he may have to make the choice himself. You could argue that any man who doesn’t know the lady he is proposing to well enough to pick a ring she will love perhaps doesn’t know her well enough to propose but in reality many men are clueless when it comes to any kind of female fashion and the lady must only hope that he asks his mother or sister for advice.
A safe option may well be diamonds with yellow gold. White diamonds being clear are unlikely to be a color she doesn’t like and are the best known gemstone for a reason. They are a classic combination either as one or a number of stones with yellow gold in a thin band. Bigger may not be better for some fiancés, large stones may be loved by some girls but some may see them as over the top and ostentatious.
Diamonds as well as being used on their own can go with other stones, popular ones include amethyst which is a purple color often associated with weddings. If she is quite a girly girl then pink stones like opals or sapphires may be her thing.
The ring itself can be in another color as well perhaps a white gold or even a rose gold or perhaps another noble metal like platinum or silver.