Diamonds are a naturally occurring mineral known for their exceptional hardness, which is a result of the strong covalent bonds between carbon atoms. This makes them highly durable and resistant to scratches and other types of damage. Diamonds are also known for their brilliance and fire, which is the result of the way light interacts with the diamond's internal structure.

Diamonds are graded based on the "Four Cs": carat weight, cut, color, and clarity. Carat weight refers to the weight of the diamond, and is often used as a measure of a diamond's size. Cut refers to the shape and angles of a diamond, and how well they interact with light. A well-cut diamond will reflect light back to the viewer's eye, creating the appearance of brilliance and fire. Color refers to the presence or absence of color in a diamond, with colorless diamonds being the most valuable. Clarity refers to the presence or absence of inclusions and blemishes in a diamond. Inclusions are internal flaws, while blemishes are external flaws.

The most widely recognized grading system for diamonds is the Gemological Institute of America (GIA) system. The GIA grades diamonds based on the Four Cs, and assigns them a grade on a scale from D (colorless) to Z (light yellow or brown). In addition to the GIA, there are other organizations such as American Gem Society (AGS), Hoge Raad voor Diamant (HRD) and International Gemological Institute (IGI) which also grades diamonds using similar criteria.