Shape Matters Most
Even before those 4 Cs (cut, color, clarity, and carat), you must know what shape your future fiancee loves. Shape indicates the actual geometry of the stone, as opposed to cut, which relates to the angles of the facets in the stone.
Settings Are Second
A quality setting -- the metal framework in which your stone is mounted -- can set the tone for a ring. A classic round engagement ring stone gets a fresh and modern makeover in a bezel setting. A trendy oval can look more traditional in a four-prong setting. Getting the right combo of shape and setting is key.
Spy on Her Style
The trend these days is for couples to ring-shop together, but if you’re more of a traditionalist and looking to surprise her with a ring she’ll love, be sure to do some reconnaissance. You can ask her best friend or sibling for help -- and swear them to secrecy. But if you’re set on not telling anyone, pay attention to the jewelry she wears. Is she more of a platinum/silver girl than a yellow-gold one? Does she gravitate to vintage jewels as opposed to simple, classic pieces? Watch her for a couple of weeks and take mental notes to size up her style.
Consider the Metal
When it comes to the band, you've got a variety of metals to choose from. One popular choice is platinum -- it's extremely durable and especially pure, making it a great hypoallergenic choice for brides and grooms with sensitive skin. There's also gold, which comes in a variety colors, including white, yellow, rose, and even green. Beyond platinum and gold, you might also consider palladium (which has a grayer hue than platinum) or even a recycled metal band, which might include a mixture of platinum and gold. Think of it as a "something old" and "something new" all rolled into one.
Buy Loose Stones
Unless you’re buying an estate ring, chances are, you’ll be looking at loose stones as opposed to stones in a setting. (The ones you see in the jeweler’s case are often just samples to give you an idea of the finished product.) The stone accounts for the vast majority of a ring’s cost, so wouldn’t you want to get the most gorgeous one imaginable (or at least in your budget)? Be sure to inspect the stone with a loupe (a handheld magnifier that most jewelers have). A good jeweler (more on that later) will be able to guide you and tell you what to look for.
Work Your Budget Well
Throw out that old two-months salary myth; You should buy the best ring you can without going into major debt. If she’s a size queen and your budget isn’t super-sized, go for a ring with a slightly larger table, or surface, area. You won’t get as much sparkle, but a one-carat ring will look much larger if the stone isn’t as deep. Don’t want to sacrifice the look of your stone? Buying just shy of the next carat (1.8 instead of 2) can equal a savings of nearly 20 percent. And when it comes to clarity, buying shy gives you the most wiggle room without affecting sparkle (often defects are not visible to the naked eye).
Always start your jeweler search with recommendations from friends and family. No leads? Check for industry organization affiliation. Stores accredited by the Jewelers of America or members of the Gemological Institute of America (GIA) are good starting places. Often large, renowned chains are reliable and offer sales. And always find out what a store’s return policy is. You want to be able to at least exchange the stone if it’s not the one she wants.
Give Yourself at Least Six Weeks
Remember that once ordered, a ring can take up to six weeks to arrive, potentially longer if you're having it custom designed. If you want to engrave the inside of the ring, be sure to request the inscription when you place your order.
Get It in Writing
Diamonds one carat or larger should be accompanied by a diamond-grading report issued by an independent gemological association such as the GIA or the American Gem Society. You might also get a "fingerprint" of your ring on the bill of sale, which would include the stone's 4 Cs, shape, dimensions, and any cosmetic enhancements. Also, anything that affects its value -- if it was made by a famous designer, is an antique or period piece, or is handmade or custom-designed -- should also be noted.
See More: Popping the Question , Engagement Rings