Buying a diamond ring can be intimidating. What do you look for? How much should you pay? Should you buy online or in a store?

Demystify the process by learning about the four C’s: carat, color, clarity and cut. This system of grading diamonds was developed 60 years ago by the Gemological Institute of America.

Then do some research online or visit jewelers. You’ll soon understand your options. Here’s a primer:

THE FOUR C’s

» Carat is a weight measurement. A 1-carat diamond weighs 200 milligrams. A ring with three small diamonds totaling 1 carat costs less than a single 1-carat stone of similar quality. There’s no ideal size. Your choice will depend on your budget and taste.

» Color is graded by letter, starting with D for rare, colorless diamonds. E and F are considered excellent, but G or H diamonds will look just as good to the naked eye. Farther down the scale, you’ll notice differences. "If you put a K color beside a G color, you’ll notice more yellow in the K," said Russell Shor, senior industry analyst for the Gemological Institute.

» Clarity measures diamond flaws, called inclusions, which might appear as tiny spots, clouds or cavities in the stone. The clarity grade SI stands for "slightly included." VS is a better grade, "very slightly included." VVS is even higher, "very, very slightly included." Most inclusions in the VVS-SI range cannot be seen by the untrained eye "unless someone tells you where it is," Shor said.

» Cut measures workmanship. The way a stone is cut enhances sparkle and luminosity and can hide flaws. The best cut rating, ideal, is rare. About a third of diamonds are rated fair, good or very good.

THE FORMULA

What should you look for in each of the C’s?

"The one thing you should not trade off on is the quality of the cut," said Shor. "Even a nice-color stone, if not well cut, will be dull and lifeless. But if it’s a middle color — like K — and it’s got a real excellent cut, it will pop and flash with all the sparkle that diamonds are famous for."

After that, "balance the color, clarity and carat weight based on your personal preference to find the best diamond for you and your budget," said Amanda Gizzi, spokeswoman for Jewelers of America.

For example, for $2,000 you might pick a 1-carat, K-color stone with a slight inclusion or a half-carat, G-color, with a very slight inclusion. An L or M-colored diamond at that price "will get you a 2-carat honker, but you’ll definitely notice the yellow and you’ll see some inclusions," said Shor.

Consumers pay $3,500 on average for engagement rings, according to Jewelers of America. Shor recommends spending at least $700 to $1,000 to get "something that’s not too small and of reasonable quality, a respectable half-carat stone."

It’s easy to compare online. At BlueNile.com set your price range, then play with carat size and the other C’s to see trade-offs.

Many websites list the four C’s for every ring they sell. Brick-and-mortar stores should be able to provide grading reports, whether from GIA or another lab.

SHAPE AND STYLE

Diamonds can be cut into many shapes. Round, the most common, offers "the biggest bang for your buck because the difference between the raw and cut diamond is smaller," said Melissa Colgan, senior style editor for Martha Stewart Weddings. But unusual shapes with retro looks and names like marquise, Asscher and pear are having a resurgence, Colgan said, partly because celebrities are wearing them.

Whether a shape is flattering depends on your fingers. "If you have long, thin fingers, you can wear something like Asscher or princess that is more square-cut," Colgan said. "If you have shorter fingers or muscular hands, marquise or oval will elongate your fingers." But long nails don’t mix with oval: "It looks like you’ve got a weird nail in the middle of your hand."

ONLINE OR IN-STORE?

Many major brands, including Macy’s, Kay, Zales, Tiffany and even Costco, sell diamonds both online and in stores.

Some retailers sell online only. Gemvara.com‘s site is fun for customizing designs. Just click to see how a ring looks with diamonds and sapphires, versus diamonds and rubies, or with white versus yellow gold.

Blue Nile has sold engagement rings to 325,000 couples over the Internet, including one for $1.5 million.

Nervous about ring size? Blue Nile will mail a free plastic ring-sizer with no obligation to buy. Blue Nile also has a 30-day return policy, no questions asked, though fewer than 10 percent of customers take advantage of it, according to company spokesman Josh Holland.

Many retail stores offer 30-day returns with no penalty as well. That’s important for surprise proposals in case the bride-to-be says no, or if she says yes but wants a different ring.

These days, though, couples often shop together.

"It’s perfectly OK to say, ‘Let’s just go and look at things together,’" said Colgan. "This way she knows what he can afford, and he knows aesthetically what she wants."

Some customers prefer online shopping so they won’t be pressured by an aggressive salesperson. But most diamond rings are bought in person, according to Jewelers of America, citing the 2011 Wedding Report, which found that only 11 percent of engagement rings are bought online.

A new online retailer called Ocappi.com offers to bridge the gap by mailing out "try on rings" made from silver and cubic zirconia. The replica rings let you see how different styles, shapes and carats look on your finger. You can order up to six replicas at once. As long as you return them, there’s no charge and no obligation to buy the real thing. Ocappi pays shipping both ways.

INVESTMENTS

In 2011 a 33.19-carat diamond owned by Elizabeth Taylor sold for $8.8 million. The same stone was $305,000 in 1968. But that’s an exception: The average diamond does not appreciate much, if at all.

Does that mean estate-sale rings are a bargain? Shor says "older stones can be a bargain," but cautions that stone-cutting has improved with computers and robotics, so older rings may not have "the quality of workmanship" found in modern diamonds.

It’s also a matter of taste.

"I think some of the old-cut jewelry is so incredibly beautiful," said Colgan. "They’re not quite as shimmery or blingy, but they have a really sweet sparkle."