HOW TO BUY A DIAMOND ENGAGEMENT RING

THE FOUR C'S

The Four C's are very important when it comes to the characteristics of a diamond to consider. 

Cut

Carat

Clarity

Colour

Many websites go indepth into this.

Each 'C' has quite a bit to consider. The challenge is to find out what's essential to a beginner and to condense it and make it as simple and easy to digest as possible.

The Diamond Buying Guide has a very good guide on the 4 C's, with alot of information.

Here it is directly from the website.

CUT

Diamond Cut

Don't confuse the diamond's "cut" with the diamond's "shape."

First, don't confuse diamond "cut" with "shape." Shape refers to the general outward appearance of the diamond, (such as round, emerald, or pear). When a diamond jeweler (or a diamond certificate) says "cut," that's a reference to the diamond's reflective qualities, not the shape (or at least it should be, we have found that even some "jewelers" don't appear to know the difference between "cut" and "shape").

The quality of the "cut" does make a difference in how a diamond looks.

Diamond cut is perhaps the most important of the four Cs, so it is important to understand how this quality affects the properties and values of a diamond. A good cut gives a diamond its brilliance, which is that brightness that seems to come from the very heart of a diamond. The angles and finish of any diamond are what determine its ability to handle light, which leads to brilliance.

(See Diamond Anatomy for an explanation of the terms used in the next paragraphs.)

As shown in the images below, when a diamond is well-cut, light enters through the table and travels to the pavilion where it reflects from one side to the other before reflecting back out of the diamond through the table and to the observer's eye. This light is the brilliance we mentioned, and it's this flashing, fiery effect that makes diamonds so mesmerizing.

 
 

In a poorly cut diamond, the light that enters through the table reaches the facets and then 'leaks' out from the sides or bottom of the diamond rather than reflecting back to the eye. Less light reflected back to the eye means less brilliance.

Good Proportions are Key
Most gemologists agree that the best cut diamonds are those that follow a set of formulae calculated to maximize brilliance. These formulae can be seen in a diamond's proportions, most importantly how the depth compares to the diameter, and how the diameter of the table compares to the diameter of the diamond.

If you opt to buy a diamond without an AGS certificate, spend some time looking at certified diamonds (where you know the Cut Grade) and train your eyes to identify the better cuts (by their "sparkle"). Cut does make a difference to the outward appearance of a diamond.

However, the variance in the proportions between anIdeal Cut and a Poor Cut can be difficult to discern by the casual observer.

Because cut is so important, several grading methods have been developed to help consumers determine the cut of a particular diamond. In general, these grades are:

• Ideal
• Premium
• Very Good
• Good
• Fair & Poor

Which Grade of Cut Should I Buy? 
Selecting the grade of cut is really a matter of preference. To make the best selection, you need to understand the various grades. Please note that the descriptions below are general guidelines.

Ideal Cut
This cut is intended to maximize brilliance, and the typically smaller table sizes of these diamonds have the added benefit of creating a great deal of dispersion or 'fire' as well. Ideal quality diamonds are truly for the person who enjoys knowing that he has one of the finest things that money can buy. This category applies only to round diamonds.

Premium
In the case of round diamonds, many Premium Cut diamonds have cuts that are the equal of any Ideal Cut diamond, though they often can be purchased at slightly lower prices than AGS Ideal Cuts. They are intended to provide maximum brilliance and fire. Like the Ideal Cut, these are also for the person who enjoys knowing that he has one of the finest things that money can buy.

Very Good
These diamonds reflect most of the light that enters them, creating a good deal of brilliance. With these diamonds, the cutters have chosen to stray slightly from the preferred diamond proportions in order to create a larger diamond. The result is that these diamonds fall slightly outside of some customers' preferences in terms of, for example, table size or girdle width, though, in many cases many of the parameters of diamonds in this range will overlap with certain parameters of diamonds in the Ideal or Premium ranges. Generally, the price of these diamonds in slightly below that of Premium cuts.

Good
Diamonds that reflect much of the light that enters them. Their proportions fall outside of the preferred range because the cutter has chosen to create the largest possible diamond from the original rough crystal, rather than cutting extra weight off to create a smaller Premium quality diamond. Diamonds in this range offer an excellent cost-savings to customers who want to stay in a budget without sacrificing quality or beauty.

Fair & Poor
A diamond graded as fair or poor reflects only a small proportion of the light that enters it. Typically these diamonds have been cut to maximize the carat weight over most other considerations.

Diamond Anatomy
Wondering what on earth is the diamond's pavillion? Table? Culet? The graphic and supporting text below explain the various "parts" of a diamond.

• Diameter
  The width of the diamond as measured through the girdle.
• Table
  This is the large, flat top facet of a diamond.
• Crown
  The upper portion of a cut gemstone, above the girdle.
• Girdle
  The narrow rim of a diamond that separates the crown from the pavilion. It is the largest diameter to any part of the stone.
• Pavilion
  The lower portion of the diamond, below the girdle. It is sometimes referred to as the base.
• Culet
  The tiny facet on the pointed bottom of the pavilion, which is the portion of a cut gem below the girdle.
• Depth
  The height of a gemstone, from the culet to the table.

CLARITY

Diamond Clarity

For clarity grades F through SI, inclusions (internal flaws) are NOT visible to the naked eye.

When we speak of a diamond's clarity, we are referring to the presence of identifying characteristics on (blemishes) and within (inclusions) the stone.

If you think about the incredible amount of pressure it takes to create a diamond and the fact that natural diamonds are not grown in a sterile laboratory, it's no surprise that mostdiamonds have flaws.

Basically there are two types of flaws: inclusions and blemishes. Inclusions refer to internal flaws and blemishes refer to surface flaws. However, in the diamond grades listed below, you'll note that none of the grades include the term "blemish" -- for the purposes of grading diamonds, all flaws are called "inclusions."

Inclusions include flaws such as air bubbles, cracks, and non-diamond minerals found in the diamond. Blemishes include scratches, pits, and chips. Some blemishes occur during the cutting processes (most often at the girdle). Diamonds with no or few inclusions and blemishes are more highly valued than those with less clarity because they are rarer.

How are diamonds graded for clarity?
Diamonds are graded for clarity under 10x loupe magnification. Grades range from Flawless (diamonds which are completely free of blemishes and inclusions), to Included 3 (diamonds which possess large, heavy blemishes and inclusions that are visible to the naked eye).

• F
  Flawless: No internal or external flaws. Extremely rare.
• IF
  Internally Flawless: no internal flaws, but some surface flaws. Very rare.
• VVS1-VVS2
  Very Very Slightly Included (two grades). Minute inclusions very difficult to detect under 10x magnification by a trained gemologist.
• VS1-VS2
  Very Slightly Included (two grades). Minute inclusions seen only with difficulty under 10x magnification.
• SI1-SI2
  Slightly Included (two grades). Minute inclusions more easily detected under 10x magnification.
  

  REMEMBER: For grades F through SI, a diamond's clarity grade has an impact on the diamond'svalue, not on the unmagnified diamond's appearance.

• I1-I2-I3
  Included (three grades). Inclusions visible under 10x magnification AS WELL AS to the human eye. We do not recommend buying diamonds in any of these grades.

F-IF	VVS1-VVS2	VS1-VS2	SI1-SI2	I1-I2-I3

While the presence of these clarity characteristics (inclusions and blemishes) do lower the clarity grade of a diamond, they can also be viewed as proof of a diamond's identity. GIA certificates include what is known as a "plot" of a diamond's inclusions -- think of it as a "diamond fingerprint." Since no two diamonds are exactly the same, comparing the uniqueness of your diamond's clarity characteristics with the plot provided on the diamond certificate offers assurance that the diamond you pay for is the same diamond you receive.

Which Clarity Grade Should I Choose?
While Flawless diamonds are the rarest, a diamond does not have to be flawless to be stunning.

In fact, until you drop to the "I" grade, a diamond's clarity grade has an impact on the diamond's value, not on theunmagnified diamond's appearance.

Diamonds with VVS and VS grades are excellent choices for both value and appearance.

More affordable (and still a great choice) are those diamonds which gemologists call "eye-clean" - diamonds with no inclusions visible to the naked eye. These diamonds are SI1 and SI2 and unless the recipient carries a 10X loupe (a strong jewelry magnifying glass), she won't see the inclusions.

As to I1-I3? Maybe when there's a diamond grade that's defined as "you can see the flaws just by looking at the diamond," nothing more needs to be said.

Okay, to be "fair" to I1-I3 -- not everyone notices visible flaws in a diamond. And not all "visible" flaws are "equally" visible -- think about the difference between dripping mustard on a starched white dress shirt and dripping mustard on a brightly-colored Hawaiian shirt (not that we think you have a lot of mustard dripping experience). Obviously, one shows up a lot more than the other -- visible diamond flaws are like that.

But if you choose to buy an I1-I3 diamond (which we don't really recommend), know that some people will look at it and immediately see the flaws -- and not just experienced jewelers.

COLOUR

Diamond Color

When jewelers speak of a diamond's color, they are usually referring to the presence or absence of color in white diamonds. Color is a result of the composition of the diamond, and it never changes over time.

Because a colorless diamond, like a clear window, allows more light to pass through it than a colored diamond, colorless diamonds emit more sparkle and fire. The formation process of a diamond ensures that only a few, rare diamonds are truly colorless. Thus the whiter a diamond's color, the greater its value.

Diamonds graded G through I show virtually no color that is visible to the untrained eye.

[NOTE: Fancy color diamonds do not follow this rule. These diamonds, which are very rare and very expensive, can be any color from blue to green to bright yellow. They are actually more valuable for their color.]

To grade 'whiteness' or colorlessness, most jewelers refer to GIA's professional color scale that begins with the highest rating of D for colorless, and travels down the alphabet to grade stones with traces of very faint or light yellowish or brownish color. The color scale continues all the way to Z.

Which Color Grade Should I Choose? 
Diamonds graded D through F are naturally the most valuable and desirable because of their rarity. Such diamonds are a treat for the eyes of anyone. But you can still obtain very attractive diamonds that are graded slightly less than colorless. And diamonds graded G through I show virtually no color that is visible to the untrained eye.

If your setting is white gold or platinum, you may wish to opt for a higher color grade than if the setting is yellow gold.

And while a very, very faint hint of yellow will be apparent in diamonds graded J through M, this color can often be minimized by carefully selecting the right jewelry in which to mount your diamond. Keep in mind that, while most people strive to buy the most colorless diamond they can afford, there are many people who actually prefer the warmer glow of lower-color diamonds.

What is Flourescence?
Fluorescence is an effect that is seen in some gem-quality diamonds when they are exposed to long-wave ultraviolet light (such as the lighting frequently seen in dance clubs). Under most lighting conditions, this fluorescence is not detectable to the eye. While most gemologists prefer diamonds without this effect, some people enjoy it. It's really just a matter of aesthetics. Learn more about Diamond Fluorescence.

CARAT

Carat Weight

A carat is a unit of measurement, it's the unit used to weigh a diamond. One carat is equal to 200 milligrams, or 0.2 grams.

Size does matter. It is not, however, a measure of your love.

Keep in mind that differences in size are clearly visible... even to the untrained eye.

The word "carat" is taken from the carob seeds that people once used in ancient times to balance scales. So uniform in shape and weight are these little seeds that even today's sophisticated instruments cannot detect more than three one-thousandths of a difference between them.

[NOTE: Don't confuse "carat weight" with "karat," the method of determining the purity of gold.]

The process that forms a diamond happens only in very rare circumstances, and typically the natural materials required are found only in small amounts. That means that larger diamonds are uncovered less often than smaller ones. Thus, large diamonds are rare and have a greater value per carat. For that reason, the price of a diamond rises exponentionaly to its size.

What Size Diamond Should I Buy? 

• First, determine your budget. One general rule of thumb when buying a diamond engagement ring is "two months salary." This is just a guideline, it's not carved in stone and your first consideration should be what you can comfortably afford -- not what the diamond industry or a jeweler tells you (they are not unbiased in this matter).
• Deciding on carat size is really about striking a balance between size and quality. If she prefers larger jewelry items, and you are working within a budget, you can still find a larger diamond of excellent quality gem by selecting one which is graded slightly lower in terms of color and clarity.
• Remember that slender fingers make small diamonds look bigger. If she has small fingers, a 1-carat diamond will look proportionately large -- and an even larger stone may appear stunningly big!
• Think about what sort of setting will hold the diamond. You'll have to be sure that the setting you choose is made to fit the carat weight of your diamond.

I'm going to try and condense this onto a design sheet, to get clued up myself on this, need to understand soemthing before you design a product for it.