Buying a Diamond Engagement Ring or Wedding Ring Know the 4 c's of diamonds before you buy
Anyone shopping for a diamond engagement ring should first acquaint themselves with "the four Cs" - color, cut, clarity, and caret. Some also include the fifth c, cost.
Color: With the exception of "fancy colored diamonds", the more colorless a diamond is, the more valuable it is.
Jewelers grade diamonds on a scale from D (colorless) to Z (light yellow). Anything more yellow than D is considered fancy and is much more expensive.
Cut: Many people make the mistake of thinking that cut means the shape of the diamond. Rather, it means the way the diamond was faceted to allow light to reflect from it. In a well cut diamond, light enters the diamond and reflects straight back to the viewer's eye. Some cutters will sacrifice cut to create the largest possible diamond, thus making too shallow or too deep of a cut and causing light to "leak" out the sides and bottom of the diamond.
Cut can be something very difficult for a layperson to evaluate, which is why it is important to get an AGS Certificate or a GIA Certificate verifying the quality of your diamond. Cut is graded Ideal, Premium, Very Good, Good, Fair, and Poor. Generally you should seek an Ideal to Very Good Diamond. Budget-conscious shoppers may opt for good.
Clarity: Diamonds frequently have inclusions, or small flaws, air bubbles, scratches, or other minerals inside the diamond. The less inclusions a diamond has, the more valuable it is, and the more beautiful it is.
The scale for grading diamond's clarity is:
F Flawless - no internal or external inclusions.
IF Internally Flawless - no internal inclusions, slight external inclusions.
VVS1-VVS2 Very Very Slightly Included - minute inclusions that are very difficult to detect under 10x magnification, even by an experienced grader.
VS1-VS2 Very Slightly Included - minute inclusions that are invisible to the naked eye and seen only with difficulty under 10x magnification.
SI1-SI2 Slightly Included - invisible to the naked eye, yet easily seen by an experienced grader under 10x magnification.
I1-I3 Included - inclusions are visible to the naked eye and affect brilliance.
Stones that are VVS2 to F are very rare and thus very expensive. Most couples will opt for something between SI2 and VS1, and will never know the difference.
Caret: This refers to the size of the stone. While some value caret above all, others prefer a small ring regardless of budget. If you're buying a ring as a surprise for someone, consider how flashy their other jewelry is. If they're not someone who wears a lot of designer labels or big jewelry, perhaps you'll want to opt for a smaller stone. This site has a very useful chart to see how the most common sizes look on a model's hand. Ideally, you'll be able to try on a variety of sizes within your budget to see which size looks best on you or your love.
Investment
While jewelers love to tell you that a diamond ring is a great investment, that's not necessarily true. Diamond's may or may not increase in value, but "antique" rings rarely cost as much as new ones do, because of wear and tear. Instead of looking at rings for their investment value, buy ones that symbolize your love, and that you'll want to wear for the rest of your lives.
Before you buy a diamond ring
Before you buy, ask the retailer to provide you with an AGS Certificate or a GIA Certificate. You'll also want them to verify that the stone(s) is not a conflict diamond.
http://weddings.about.com/cs/rings/a/diamondfourcs.htm
