How a stone is cut is important, not only forming the shape of a diamond, but how well the light reflects and refracts off the facets. The better the cut, the more lively a stone will look (given the diamond is not littered with inclusions giving it a solid white appearance; if the light can't reflect, then the cut really isn't useful due to quality issues.) Also, the shape of a diamond is also important. Some shapes are more desirable than other's (in style) during certain periods of time. This can also change the value of a stone depending on it's demand. Want to play it safe with a diamond holding better value? Go with the traditional round stone.
A guide to understanding how quality can affect pricing of diamonds:
Two common, yet very important, facts about diamonds that people often misinterpret are:
1. Diamonds are the hardest "NATURAL" mineral on Earth: Most people get this confused with saying that "a diamond is the hardest substance on Earth," forgetting the term "Natural"; The fact is that there are many man-made substances that are much harder than diamonds which can crack or chip these beautiful stones with ease or enough force.
MORE IMPORTANTLY HOWEVER:
2. Diamonds are not all equal. Many people can be under the impression that they can save money by purchasing a diamond of the same size (or any one of the "same" properties of stone) from one place over another and can be considered a "deal." The fact is that, though a diamond can have the exact same property and same size as another, there are other properties that can also affect cost. For example, a 1 carat diamond with H color can still be cloudy or littered with inclusions (grade: I) while another can be mostly clear (SI and better). This difference can make a significant difference in cost; however, you are also getting a much better quality stone which will also glisten with brilliance much more abundantly.
Below is information to help you become more knowledgeable with diamonds to better assist you with shopping for diamonds. Remember, all of the properties below play important roles in a diamond's value; not just one.
Here is a quick reference chart to identify the different parts of a diamond: Girdle, Table, Crown, Culet, and Pavilion. Not labeled are the facets, (the smaller many triangular parts shown in the chart that play an important role with the reflection and refraction of light.)
Depicted above is a clarity chart. "Clarity" measures the density or amount of imperfections in a stone. I1, I2, and I3, are all defined as being able to see any imperfections with the naked eye. SI2 and better graded stones are defined as needing magnification of 10x or better to see any imperfections. The amount, or visibility, of imperfections under magnification are then graded depending on the ease of spotting, size, and location. Remember, the "I's" are visible inclusions with the naked eye, SI2 or better are only visible under magnification. Clarity is important not only for appearance of imperfections, but also can play a major role in the reflecting and refracting of light through the facets which causes that brilliance.
Just as important as Clarity, Color also plays a major role in diamond grading. Unless noted as a fancy colored stone, diamonds are graded from "D" (colorless) descending through the alphabet to "Z" (light to strong yellow.) Color grading of a diamond is defined as "depth of color," a combination of tone (darkness or lightness) and saturation (intensity). Once again, depending on how "colorless" a diamond is will also affect the price of a diamond.
Carat-Weight is believed, by many, to be the determining factor of what a diamond's value is. This belief is partially true, and partially misconstrued. When a diamond is priced, Carat weight is taken into a good amount of consideration.....as well as all of the other previously mentioned properties. Remember, when determining the value of a Diamond, the entire combination of quality determines the value. You can have a Larger stone be more affordable than a smaller one if the other properties are more desirable than that of the larger diamond. The better overall quality a diamond more rare.... ...and more desirable.