DIAMOND CARAT WEIGHT
Carat weight is the weight of the diamond measured in carats. One carat is divided into 100 "points," so that a diamond of 75 points weighs 0.75 carats. Carat weight is the most obvious factor in determining the value of a diamond. But two diamonds of equal carat weights can have very different prices, depending on their quality. Diamonds of high quality can be found in all size ranges.
The scale above depicts the carat weights and diameter sizes of round diamonds. Other diamond shapes will vary. For example, while a 1 carat round shape diamond is 6.5mm on average, a 1 carat princess shape diamond is often 5.5mm instead.
Since diamonds are measured by carat weight, and not size, two diamonds of the same weight could have different sizes. Why? If a diamond is cut too shallow it will have a larger diameter but a smaller depth. Likewise, if a diamond is cut too deep (or bottom heavy) it will have a smaller diameter and thus appear smaller when viewed from the top.
Cut is very important. A properly cut diamond will both yield a good face up size and sparkle well. For example, let's say you're interested in a 1.00ct round diamond. A poorly cut diamond that is bottom heavy may only have a diameter of say 6.1mm. So not only are you short changed on the face up size, but you get a diamond that was not cut to it’s full potential and thus does not sparkle like it should.
This is why it’s important not just to get the biggest diamond you can afford, but the best cut diamond as well.
CLARITY
Diamond Clarity is a measure of a diamond's internal flaws and impurities. A diamond that is virtually free of interior or exterior inclusions (commonly referred to as flaws) is of the highest quality, for nothing interferes with the passage of light through the diamond. To determine a diamond's clarity, it is viewed under 10x magnification by a trained gemologist. Small inclusions neither mar its beauty nor endanger its durability.
Flawless - No internal or external flaws.
Internally Flawless - No internal flaws. Very rare and beautiful diamonds which are quite expensive.
VVS1, VVS2 - Very, Very Slightly Included: Very difficult to see inclusions under 10x agnification. An excellent quality diamond.
VS1, VS2 - Very Slightly Included: Inclusions are typically not visible to the unaided eye. Less expensive than the VVS1 or VVS2 grades. Even properly graded VS2 diamonds can have visible inclusions in very large sizes or diamonds that do not have many facets which make flaws more visible such as an emerald shape.
SI1, SI2 - Slightly Included: Inclusions are typically visible under 10x magnification, SI clarity diamonds are usually of the best value since inclusions are not always visible to the naked eye and prices are reasonable when compared to prices of the VS2 clarity and up grades. Some SI clarity diamonds do have visible inclusions and should be evaluated carefully.
SI3 - Slightly Included-Included: SI3 is an EGL grade and is not recognized by GIA or other gemological institutions. The SI3 clarity grade has been adopted by other labs as well. Typically an SI3 clarity grade would usually be given an I1 grade by the GIA.
I1 - Included: an I1 clarity diamond will have a visible flaw which can be seen with the naked eye, but there should only be one major flaw which is not too obvious. Overall the diamond should still look nice however I1 clarity grades vary quite a bit so one should be very careful when making a selection. If your budget only allows for an SI3 or I1 clarity diamond, be sure to examine it thoroughly before purchasing and make sure the inclusions do not detract from the overall beauty of the diamond.
I2, I3 - Included: These diamonds will have inclusions visible to the naked eye, and will have many black spots from the black inclusions and/or appear very cloudy or milky from the white inclusions, feathers and cracks.
COLOR
The diamond color grading scale varies from totally colorless to light yellow. The differences between one diamond color grade and another are very subtle and difficult to distinguish.
D-F : Colorless: perfect or almost perfect color.
G-J : Near colorless: good to very good color, and this diamond may "face up" colorless when mounted, especially in white gold or platinum.
K-M : Light but noticeable yellow or brown tint. Not recommended for larger diamond stone purchases, especially in engagement rings.
Out of the 4 C's, color and cut are the two most important characteristics of a diamond. Do not compromise on color. An "H" color or better is usually best.
DIAMOND CUT
Many people confuse diamond cut with the shape of a diamond. Unlike shape, diamond cut is the metric used to evaluate the precision and quality of the overall cutting itself. Diamond cut is the most complex of the 4 Cs, and not even the major labs agree on how to best measure the quality of a diamond’s cut. Diamond Cut usually takes into consideration 3 criteria: Proportions, Symmetry and Polish. Sometimes Light Performance or other tests can be used to also calculate the precision of the diamond cut.
| 1. When a diamond is cut to proper proportions, light is reflected from one facet to another and then dispersed through the top of the stone similar to the image shown to the left. |  |
| 2. If the cut of the diamond is too deep, some light escapes through the opposite side of the pavilion. |  |
| 3. If the cut is too shallow, light escapes through the pavilion before it can be reflected. |  |
Up until several years ago, evaluating the cut of a diamond was not an easy task. Dimensions and proportions of the diamond as well as its finish (both symmetry and polish) were all the information able to be evaluated, thereby making it difficult for consumers to understand what is a well-cut diamond and what is not.
Symmetry measures how well the facets match the other facets. A diamond cut with very good or excellent symmetry means that if you were to look at all the facets and compare to one another, they would look very uniform with very little variance. This doesn't mean however the diamond is not cut too deep or too shallow.
Polish is a measure of the surface finish. If a diamond is only polished "Fair" or "Good" it does not allow maximum transition of light through the facets, due to it's "rough" surface texture. Very Good polish by most lab standards is usually sufficient to get the top cut grade, and excellent/ideal is often preferred. Polish graded above Very Good is not detectible to the naked eye.
So while symmetry and polish help you determine if the diamond is cut well, it doesn’t always mean the diamond will perform like a well cut diamond.
Recently both the GIA and AGS have adopted “Cut Grades” which factor all these things together; the proportions, symmetry and polish, and sometimes even light performance. Currently, GIA issues a cut grade on its round diamonds, and AGS issues a cut grade on round and princess shape diamonds they grade.
We often recommend consumers consider at least a GIA Very Good or Excellent Cut grade diamond if considering round or princess shapes. For those who want to be assured of a top cut, there is also the coveted AGS Triple Zero grade which they claim to be the best ideal cut achievable. It's hard to go wrong with either an AGS Ideal grade or GIA Excellent grade stone.
SOURCE
WIKIPEDIA
DIAMOND INFO
THE DIAMOND BUYING GUIDE
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