The Rock Chart - Know Your Diamonds

The 4Cs are a rock group. And the rock they sing about is the diamond.

The four - carat-weight, clarity, color and cut - are the essentials you need to know to select engagement or wedding jewelry.

No two diamonds are alike, points out the Gemological Institute of America, and a stone's rarity and cost are determined by its unique characteristics as measured by the 4Cs. Size alone doesn't make a diamond more valuable, GIA says; using all four criteria, a smaller stone of exceptional quality can be more valuable than a larger diamond of lower quality.

Here's a 4C primer:

arat-weight. Diamonds are weighed in metric carats; a carat weighs the equivalent of a small paper clip. The carat is expressed in terms of points, and there are 100 points to a carat. According to the Diamond Information Center, an arm of diamond producers De Beers, fewer than 5 percent of all gem-quality diamonds are larger than one carat.

larity. Formed naturally over millions of years, diamonds typically include small traces of non-crystallized carbon, the element from which they were formed. You probably won't be able to see these inclusions (internal) or blemishes (external), but GIA's diamond grading system will rate stones from flawless (F) to those with more prominent imperfections (I3). The more flawless the stone, the more costly it will be.

olor. This is the most subjective of the 4Cs, says the DIC. Most diamonds appear white, but almost all have barely perceptible tints of color. Gemologists and jewelers grade color on a letter scale starting with D (colorless), moving through the alphabet to Z (light yellow or brown). D, E, and F color-graded stones are more expensive because of their rarity. But well-cut diamonds with good clarity in all color grades can still dazzle, playing on the stones' unique personalities. In fact, some rare diamonds, called "fancies," have strong, pure color and can be found in pink, blue, canary, and other colors.

ut. It's not the shape but the skill of the jeweler that counts in the cut and is the most important of the 4Cs, says the DIC. A good cut releases the gem's "fire" or brilliance through the proportion of 57 or 58 facets (angled planes) that allow the most light to be reflected through the stone. Diamonds that are cut too shallow or too deep lose light that spills through the side or bottom. Jewelers can cut diamonds in all sorts of shapes: round (brilliant), pear, emerald (rectangular), marquise (oblong with pointed ends), princess (square), oval, and heart. Both organizations stress dealing with reputable jewelers and asking for a diamond grading report on the stone you may be buying.