Diamonds are the most valuable of all gemstones because of their rarity. To produce a single one-carat diamond, 250 tons of earth will be mined! This makes diamonds the most desired gemstones in the world. Because not all diamonds are created equal, the Gemological Institute of America created a diamond grading system to objectively evaluate diamonds based on the four C's: Carat, Color, Clarity, and Cut. Below is information directly from the GIA regarding diamond grading and evaluation.
Diamonds are weighed in metric carats and each carat is equal to 100 points. A .75 carat diamond is the same as a 75-point diamond or a three-quarter carat diamond. It's not just a diamond's carat weight or size that makes it valuable, but a combination of the four c's that make a diamond valuable.
Diamonds are valued by their lack of color – the less color, the higher the value. They are graded on a range from D to Z, with D being colorless and Z being an undesirable yellowish color. Letters D to F are considered colorless, G to J are considered near colorless, K to M are faint yellow, N to R are very light yellow, and S to Z are light yellow.
Since diamonds experience extreme heat and pressure when they are formed deep within the earth, most diamonds have internal and external characteristics known as inclusions. It is extremely rare to find a flawless diamond that lacks these inclusions, but they are not always visible to the naked eye. The grading scale for clarity ranges from flawless (F), to very very slightly included (VVS), very slightly included (VS), slightly included (SI) to included (I).
The cut of a diamond promotes its brilliance and refers to the diamond's proportion, symmetry and polish. An understanding of diamond cut begins with the shape of a diamond. The most popular cut of a diamond is round brilliant cut, but other shapes such as princess, emerald, radiant, marquise, oval, and pear are also choices when choosing a diamond. The GIA cut scale ranges from excellent, to very good, good, fair, and poor.