The easiest C to understand, carat weight refers to the weight of a single diamond or the total weight of all diamonds in a piece. One carat is equal to one-fifth (1/5) of a gram, a fuzzy measurement to most of us on this side of the Atlantic. Consider this perspective instead: If you stuffed 141 one-carat diamonds into an envelope, their combined weight would require a one-ounce first-class stamp.  

Each carat is divided into a hundred points, just as one dollar is divided into one hundred cents. That is, a quarter-carat diamond is equal to twenty-five points, just as a quarter is equal to twenty-five cents.

If you want a bigger look for the same carat weight, consider an oval diamond instead of a round. Ovals are brilliantly cut like rounds but carat-for-carat, appear larger than rounds to the naked eye.

When buying loose stones, consider a diamond that's just shy of a price inflection point. That is, rather than choosing a stone that weighs a carat or slightly more, find a stone that weighs just under one carat. The lighter stone's per-carat price could be 10% less. Apply the same concept to other inflection points such as a half carat, two carats, etc.

Diamonds that are purely colorless are exceedingly rare and valued. The vast majority of diamonds have a bit of color in them, which may or may not be visible to the naked eye. Most common is a tinge of yellow or, less commonly, a trace of brown.

Diamond color grading at the GIA, inventors of the grading system and the 4Cs teaching tool, begins alphabetically with D and ends with Z. Beyond Z, other systems assign grades to diamonds with color deep enough for them to be considered colored diamonds.

Color grading at GIA is exacting and standardized. As with clarity grading, GIA and other grading institutions offer the most accurate information on individual stones, that is, diamonds that are not mounted or set in jewelry.

GIA grades color in groups, recognizing the variation in perception between observers. Diamonds that color grade as D, E and F are termed colorless, that is, no color is observed by their methods. Near-colorless diamonds grade as G, H, I and J, their color unapparent to most observers. Diamonds that show a faint tint of yellow or brown grade as K, L, M and N. Very light yellow or brown tints visible to the naked eye range in color grades O through U. Grades V through Z are light in color with yellow to brown visible to the naked eye even when mounted in jewelry.

The best way to use this information is to apply a few basics to your situation. First, if you prefer diamonds in a yellow gold setting, you might choose diamonds in the K − M range. The slight warmth of the diamond's color will be quenched in part by the yellow gold mounting, making the diamond appear whiter to the naked eye.

Conversely, if you prefer white metal mountings in white gold or platinum, consider choosing cooler and more colorless diamonds in the D − J range, as your budget permits.

Diamonds in the G − J range appear colorless to casual observers in most settings and can be your best buys. Consider the importance of the piece, how and where it will be worn as well as your budget to help you decide.

Color can be an important C when choosing certain diamond styles and shapes. For example, the faint yellow tint in an L – M graded round brilliant may not appear as noticeable to the naked eye as that of an L – M graded diamond shaped as an emerald-cut. See more about this in the next section.

Observe diamond jewelry in different types of lighting. See that the diamonds' level of color or colorlessness matches throughout the piece. Be sure there isn't an off-color stone in the setting that disturbs the balance of the piece. Remember that you might find an off-color stone acceptable if it's in a less visible part of the setting, or the price is right.

GIA grades clarity range from flawless (F) to included (I). Large size flawless stones are generally in museums or private collections. After flawless, the grades are:

Internally flawless (IF)
Very, very slightly included (VVS, further divided into VVS1 and VVS2)
Very slightly included (VS, further divided into VS1 and VS2)
Slightly included (SI, further divided into SI1 and SI2)
Included (I, further divided into I1, I2 and I3)

You can learn more about how different grades are determined on the GIA Web site or elsewhere. Try not to get lost in details that don't apply to your situation and remember to stay focused on a diamond's entire profile and your own wishes.

Here are a few tips we think you'll appreciate knowing:

Emphasize (and pay for) higher clarity grades when it matters. Most people desire better clarity in an engagement ring's solitaire diamond than in a tennis bracelet or stud earrings.

Most commercial diamond jewelry falls within the SI (slightly included) range. That is, most diamond jewelry found at department stores, shopping malls and average independent retailers.

Discount diamond jewelry commonly ranges from SI (slightly included) to I (included) ranges. Discounts happen for a reason; this is one of them.

VS (very slightly included) diamonds provide excellent value for those who can afford them. All other Cs being equal, that is. A VS diamond with poor color or a lifeless cut is not a bargain.

SI (slightly included) diamonds that are well cut and show good color provide good value. Don't let the SI designation keep you from choosing a piece of diamond jewelry you like that shows great style and bright sparkle at a good price.

Inclusions may be less visible in brilliant-cut stones than in step-cut stones such as emerald cuts. Factor this in when you balance clarity in your 4C equation.

This is the most difficult C for the untrained person to assess. Cut refers to a diamond's measurements, proportions and angles, all of which contribute to a diamond's fire, brilliance and sparkle. In essence, cut relates to a stone's beauty, liveliness and desirability.

Despite GIA's recently announced cut grade system after years of research involving tens of thousands of diamonds using advanced scientific and observational studies, cut remains an area of controversy and refinement among professionals and gemological societies.

Of the many important conclusions that emerged from examining a variety of cuts, one holds special relevance to the potential diamond buyer: No single cut defines a standard of beauty all observers found superior or desirable. Within certain parameters, a diamond's beauty is in part in the eye of the beholder.

One reason for this is that we perceive and place different levels of importance on fire, brilliance and sparkle. Fire refers to the flashes of rainbow colors seen in a diamond's glint. Brilliance refers to the amount and whiteness of light returned to the eye when you look at a diamond. Scintillation, or sparkle, refers to the flashes of light seen when you or the diamond moves. Which do you like best? Does your mate feel the same way?

Understand that your eyes may appreciate these three special diamond characteristics differently than your jeweler, your spouse or the anonymous grader who came up with numbers and percentages in the diamond comparison table you're reviewing.

Cut is not the same as shape. Cut refers to the way the diamond was angled and proportioned. Shape is the form of a diamond's outline, as in round, pear, emerald, cushion, etc. Shape is a matter of personal preference and style. Cut pertains to the skill of the cutter and how well the diamond's cutting approximates an ideal or standard that maximizes fire, brilliance and scintillation.

Brilliant cuts are far more sparkly than step cuts such as emerald cut stones.

If your preference is for a more sedate and less showy look, consider step-cut diamonds such as baguettes or emerald cuts. You may place more importance on color and clarity when choosing a step cut diamond. 

If sparkle and brightness are your idea of diamond beauty, look for brilliant cuts in the shape you prefer, that is, round brilliant, pear, marquise, princess or others. Ask your jeweler to show you different diamond shapes to help you judge what looks best on your finger, lobe or neckline. Value shoppers may emphasize cut and carat weight over color, clarity or both when choosing a brilliant cut stone.

Diamond companies, writers and gemological societies have many opinions when it comes to the Fifth C, which is an unofficial C anyone can invent, defend or trumpet. Examples include Confidence, Conflict-free, Certification and Cost.

While these are all important, our Fifth C candidate is simply: Cleanliness.

Diamonds are notorious grease magnets that easily attract skin oils, perspiration and dirt. We are loathe to remove our diamonds before activities such as showering, swimming and hand washing, not to mention when we apply lotions and hand creams. Over time, this grime and grease build up will rob a diamond of its original beauty, dull its brilliance and quench its sparkle. Grime can make a high color and clarity diamond appear lifeless.

Think about clean when you're considering a pristine VVS1 center stone to mark a momentous occasion or when bargaining for the best possible price on a G – H color tennis bracelet. If you're the fortunate diamond recipient, honor the intentions and effort of your gift-giver by keeping your diamonds clean and lively. If you're the gift-giver, give thought to how your intended cares for jewelry and how that might affect your purchase. Not very romantic perhaps, but practical nonetheless.

If your budget doesn't permit you to buy the stone you desire at this time, remember that a diamond will always look best when it's well cleaned. Encourage your intended to clean diamond jewelry regularly. It's easy and can be done in the time it takes to brush your teeth. Do it – regularly. For tips on cleaning diamond jewelry, click here.