Men know they can't afford to take chances these days.

They generally don't shop alone for an engagement ring. They go to the store with their girlfriend. She tells them what she wants. They obey.

Many times, the man will return alone and buy it, says Jennifer McCullough, merchandise manager at Smyth Jewelers.

Couples have this part of the jewelry routine down. But they sometimes don't know much about diamonds.

"Get educated first," advises Laura Simanton, senior public relations manager for the Gemological Institute of America. "Nobody wants to be uncomfortable when they go into the store. People don't mind plunking down the money. But they want to get a good value."

First, decide on a budget. It was once standard practice to instruct men that they should spend the equivalent of two months' salary on an engagement ring. That doesn't happen anymore. "They spend what they want to spend," says McCullough. "I would probably say most people spend more than that, but there is no standard rule."

If someone can spend only $400, jewelers will work with that person. "There's going to be certain things you give up for your price point. You may have to choose a simple mounting and put money in the diamond. There are steps you can take," McCullough says. "We can start you anywhere."

Diamonds are available everywhere these days—from the big-box store down the street to the Internet to the local jewelers. "What we tell people is to choose a qualified jeweler," says Simanton. "It would be like selecting a doctor and dentist. Who has the greatest experience? Who is a professionally trained gemologist?"     

Considering the investment, it makes sense to buy from someone who understands the diamond's cracks and flaws, Simanton says. "'Is it natural? Is it synthetic? Is it treated?' These are the things you need to know."

But there's more: The best jewelers will stand behind the product, says Nola Dobratz, marketing director for Radcliffe Jewelers. "We'll be there to size it," she explains, "and if anything goes wrong, that's what we're here for."

Study "the four C's" of choosing a diamond—cut, clarity, carat [weight], and color. "The four C's" started "as a way for instructors to talk to students," says Simanton. Now it is a way to talk to consumers as well. "You can walk into any jeweler anywhere and say, 'Tell me about this.'"

Armed with this information, consumers can comparison-shop. "Once you've got the jeweler you trust and you've got 'the four C's' down," Simanton says, "get an independent diamond grading report."

The report can shed light on the diamond's value. For example, it is possible to radiate or heat a diamond to enhance its color, she explains. The jeweler's grading report should give the consumer that information. It will describe the ring, Simanton says, as, for example, "a one-carat diamond, round-cut, that has little flaws invisible to the naked eye. It has probably been heat-treated to bring up the color."

Rings designed in "micro pavé" are popular now. With this design, many smaller diamonds surround the center stone. "It is very pretty and gives more life to the stone and makes it look larger than it is," says Dobratz. "It's very elegant."

Several years ago, couples were choosing an assortment of precious gemstones as centers for their engagement ring. Some women prefer sapphires, rubies, or emeralds. Famously, the engagement rings of the late Princess Diana and Heather Mills (now ex-wife of Paul McCartney) were sapphires. Both were large and exorbitantly expensive. But for the most part, sapphire rings are less expensive than diamond.

But other stones still aren't as popular as diamonds, says McCullough. "I think diamonds have a romantic aura. A diamond symbolizes love. That's why most people want the diamond."

The next step—buying the wedding band—is sometimes put on hold. "I think couples get down to the end and they realize that weddings are not inexpensive," McCullough says. As an insider in the jewelry business, she didn't make that mistake. "I worked here, so I got the engagement ring and the band immediately." Manufacturers can discontinue a style with no prior notice, McCullough explained.

Once, couples wanted matching wedding bands. But that's not the trend today. "Sometimes yellow gold looks better on one person and white gold on the other," says Dobratz. 

Couples should shop for wedding bands four to six months early. "We'll have time to get the ring in and not have to rush it," McCullough says. The ring can be adjusted if necessary. Many couples also engrave the ring with initials, the date of the wedding, or a specific word or phrase that has meaning to them.

The average cost for an engagement ring is $5,000 to $10,000 with diamond and mounting, says McCullough. Wedding bands for women cost $700 and up. Men's rings are less expensive—$300 and up.

When buying wedding jewelry, couples should also budget the cost of gifts for the bridal party—usually about $40 to $100 per attendant, McCullough says. "It depends on the wedding. The bride may choose to spend more."