In the 1940s, the Gemological Institute of America (otherwise known as the GIA) created a diamond-grading system based on the four Cs of diamonds: cut, color, clarity, and carat. Today it remains the standard by which diamonds are judged internationally. Cut refers to a finished stone's proportion, polish, and symmetry. During the cutting process, artisans sculpt tiny angles -- known as facets -- into a raw stone, ultimately creating its overall shape. Poorly placed facets result in dullness, while those with optimum measurements guarantee that distinctive glitz and glimmer. Diamonds are carved into a variety of silhouettes, but "80 percent are round brilliants," says Andrew Coxon, president of the Diamond Institute for De Beers Diamond Jewelers in London. "They have the most sparkle and are also the most popular among consumers." Rough diamonds provided by Rough Diamond World.

Other common silhouettes include pear (Richard Burton gave Elizabeth Taylor one that weighed 69 carats), marquise, emerald (Prince Rainier proposed to Grace Kelly with this rectangular shape), Asscher, cushion (the world-renowned Regent diamond is a whopping 140-carat version), princess, and heart. In 2006 the GIA introduced a system for rating round brilliants -- ranging from "poor" to "excellent." Fancy shapes aren't held to the same standards, but you'll know a well-cut stone when you see it. Look for brilliance (it reflects light), fire (it flashes colors), and scintillation (it sparkles). And when shopping for an engagement ring, be sure to examine stones in a variety of different lighting environments, suggests Tom Moses, senior vice president for GIA Laboratory & Research in Carlsbad, California. On the next several slides see examples of all of the diamond cuts.

There are two categories: colorless and fancy. Colorless diamonds are graded on a universal scale from D (completely clear) to Z (traces of yellow, gray, and brown), with a letter grade for each shade. While D color is like looking into a piece of glass, "E and F are also in the colorless range, and G through J are near colorless," explains Moses. "After K or L, colors start to turn very light yellow." Completely colorless stones are typically the rarest and most expensive on this scale, but some consumers actually prefer the look of a slightly warmer (or even very warm) white. For that reason, most diamonds sold in jewelry stores lie in the D to L range, as variations can be indiscernible to the untrained eye, and choosing a stone often falls to personal reference.

Fancy diamonds (yellow, pink, blue, and other naturally colored stones) are rarer, often putting them at a higher price point than colorless ones. While the GIA doesn't have a rating system in place for fancies, a more saturated shade is generally a more expensive stone.

Most diamonds contain two types of naturally occurring imperfections: internal flaws (inclusions) and surface flaws (blemishes). Taking these traits into consideration, each diamond is given a clarity grade from the GIA's 11-step scale: Flawless (FL), Internally Flawless (IF), Very Very Slightly Included (VVS 1 and VVS 2), Very Slightly Included (VS 1 and VS 2), Slightly Included (SI 1 and SI 2), and Imperfect Included (I1, I2, and I3). Though flawless stones exist, they're incredibly rare -- most jewelers have never even seen one. As a result, many jewelry stores sell stones in the VS to SI range, which are considered "eye clean." The inclusions in this range are visible only under 10x magnification, says Moses. And though it may seem counterintuitive, a microscopic inclusion or two can actually be a good thing. "Like fingerprints, they make your stone unique and can help identify it," he says. Stones in the I range, however, should generally be avoided.

Not to be confused wi th karat, which is a measure of gold purity, the term carat refers to a diamond's weight, not its size. Each metric carat weighs 200 milligrams, and each carat is further divided into hundredths, or points. Sound complicated? Jewelers often make this simple analogy to money: Just as a dollar contains 100 pennies, each carat comprises 100 points. A 75-point diamond weighs 0.75 carats, a 50-pointer is 0.50 carats, a 25-pointer is 0.25 carats, and so on.

Unlike the other three Cs, carat weight isn't always a direct reflection of cost. "If a diamond cutter chooses to leave in a flaw, the stone will be bigger but less brilliant and less expensive," says Moses. Along the same lines, a large stone with a low color grade can be less expensive than a smaller one with a high grade. It boils down to a matter of preference: If size matters but budget is a concern, you might opt to downgrade in quality. When shopping for a diamond, remember that stones weighing in at just under the full carat mark are much less expensive than stones right at the full carat mark. "A 95-point diamond costs less than one that weighs a full carat -- but when the stone is set, you won't be able to tell the difference in size," Moses says.

Set a budget before you start shopping. "Ask to see only rings you can afford, and make your decision from those," says Sally Morrison, director of the Diamond Information Center in New York City. To understand what goes into the price of a diamond, remember that cut, clarity, carat, and color are all factors of a diamond's grade, and therefore, price.

Dirt, lotion, and even skin's natural oils can coat and cloud your ring, but with a little TLC, it will continue to dazzle. Once a year, take your ring to the jeweler for a thorough cleaning and a prong checkup. To spruce it up at home once a month, soak it in a mixture of warm water and mild dish washing liquid before scrubbing the stone gently with a soft toothbrush.

Also, make sure to get your ring appraised by someone certified by a recognized association such as the Gemological Institute of America, and insure it.

If you saw the movie Blood Diamond, the mere mention of the diamond industry may evoke images of violence and greed. Ten years ago, the trade of conflict diamonds -- stones sold to fund armed conflict in war-torn African countries -- was a major concern. And while a percentage of these stones are still traded, especially in areas such as Zimbabwe, steps are being taken to help eradicate the problem. In 2003 government representatives, the diamond industry, and civil-rights workers in Kimberley, South Africa, created the Kimberley Process Certification Scheme (KPCS), a regulatory system. Its mission is to prevent conflict diamonds from being traded. Before KPCS went into effect, about four percent of diamonds on the market were "blood diamonds." Now, thanks to new regulations (KPCS requires that all diamonds be transported with conflict-free documentation), that figure is less than one percent. M ost major design houses and boutiques abide by the KPCS regulations, though not all diamonds come with conflict-free certificates. Always ask the seller for assurance.