Choosing the Band
1 Choose the right metal for the band. The band is the circular part of the ring that sits around the finger. It's usually made from gold, silver, or platinum, although it can be made from some other combination of metals. Note that, since metals rub and wear on one another, you should only wear rings of the same metal next to each other. Look at her existing jewelry collection to get an idea of what she prefers, differentiating from the three most common metals:
Gold is yellow-orange in its natural state and is normally alloyed with another metal for durability. Some prefer the lighter yellow color of less pure gold such as 14K or 10K, as the yellow reflection will slightly tint the diamond.
White gold is gold alloyed with metals purposely to give the mixture an off-white color, it and must be plated with another metal--rhodium is common--for a bright silvery appearance. This plating wears off -- but some jewelers will replace their rings from time to time for free if you ask in advance.
Platinum is hard, strong and naturally silvery, but will dull slightly over time with wear, which is not necessarily a problem. Since a gold setting will make a diamond look very slightly yellowish, it is a waste of money to buy a very high color grade diamond to put in it.
Silver is less common, but also less expensive. It is usually alloyed with something else to hold strength and shine.
2 Choose the right style band for your partner, from fresh and funky to simple and traditional. Once you know the right metal, you still have some options with the band. You often have a bewildering number of choices -- from classic, plain bands to twisted, wrapped, or other unique band designs.
Do you want other stones inset into the band or just metal?
Do you want a thick band showing off more stones/metal, or a thin band?
3 Consider the "setting," or how the stone will be set in the band. The setting refers to the piece that holds the gemstone in place on the band. The setting can be "pronged" or "invisible". A setting with a bezel, or at least six prongs for some redundancy, is safer for a ring that is worn during significant activity, though some women may prefer less metal in the way of their stone.
4 Choose the right size. Choosing the ring size of the band is an important part of choosing the right engagement ring. One way you can figure out her ring size is by sneaking out one of the rings she wears a lot to have a jeweler figure out the size -- as long as she doesn't have time to miss it. If you can't get a ring out, try:
Tracing the inside of the ring on a piece of paper, then using that for sizing.
Placing the ring on your finger, then marking with pen or sharpie how far up it slides.