It’s the old rule of diamonds, the one that is probably most commonly known: the four C’s. That stands for cut, clarity, color and carat.

But it may be more complicated than it seems.

Patrick Hutchins, a certified diamontologist with Hurst Fine Diamonds, 3140 Iowa, says his most frequent customers are not the people who will end up wearing the jewelry, so the selection process can be a little tricky.

“Hopefully, the guy has done enough research or listened well enough that he knows what she likes,” Hutchins says.

He advises buyers to observe a diamond carefully to assess its quality, and to have an in-depth conversation with the vendor.

“I want to make sure that the customer looks at the diamond very slowly,” Hutchins says.

Cut

What makes a diamond beautiful is its sparkle, and what makes a diamond sparkle is its cut.

“Never sacrifice cutting,” says Rich Yeakel, of Marks Jewelers, 817 Mass. “Cutting is what makes a diamond so radiant.”

Diamonds are cut in a way that maximizes the reflection of light through the stone and back to the surface, creating the sparkle. Depending on the depth, symmetry and proportion of a cut, the reflection of light can be altered.

A diamond is cut using a series of facets to direct the light. A round brilliant cut, which has been the most common in recent years, consists of 57 facets.

Ruth Batson, executive director and CEO of the American Gem Society, says a diamond can only be fully judged by looking at it in person.

“Pictures kind of help illustrate when we’re talking about cutting, but cutting has to do with the depth percentage of the stone,” she says. “Ask for a grading report with a cut grade.”

The shape of a diamond is also part of the cut. The round brilliant shape is one of the most common, but a number of other shapes have become popular in recent years. Fancy cuts, referring to all shapes beyond the circular norm, include princess, marquise, pear, baguette, briolette and heart cuts.

Hutchins says the shape of a diamond is a matter of personal preference, but reflection and sparkle are determined differently for each shape.

Clarity

Only about 20 percent of diamonds are used as gemstones, and the reason relates to their clarity.

“Most of the time, a diamond has things stuck in it to the point that it can’t be used,” Hutchins says.

Diamonds are normally graded on a scale from flawless to imperfect, with various ratings in between. Flawless denotes the highest quality, but other grades vary in terms of how the stone’s appearance is affected.

Yeakel says an imperfect diamond will have impurities that are visible to the naked eye.

“You’re looking for impurities in the stone. We call those flaws or inclusions,” he says.

Some flaws are visible to the naked eye; others require magnification to be noticed.

Color

Crystal clear diamonds aren’t necessarily the most common.

Diamonds can be found naturally or artificially tinted in a range of colors from blue to pink, but the majority of diamonds are actually yellow or brown.

“Most diamonds are yellow in color,” Yeakel says. “It’s more rare to have a colorless diamond.”

Diamonds are often graded on color using a scale from D to Z, D being colorless and Z being obviously colored. On this scale, D through F are colorless, G through J nearly colorless and K through M slightly colored.

Hutchins says diamonds ranked below M are often less desirable due to their earthy hue. Colorless diamonds usually cost more because of their rarity. Diamonds that are naturally colored in other ornamental hues, such as blue or pink, are often more expensive, as well.

Carat

The weight of a diamond is measured in carats, which is then broken into 100 points.

A larger diamond is more expensive. However, due to the rarity of diamonds beyond a single carat, the price per carat of larger diamonds increases rather drastically.

Hutchins says the main factor in selecting the size of a diamond is simply the amount of “bling” desired by the customer. The aesthetic appeal of a diamond comes from the other characteristics, and the size is often a separate matter.