The way that a diamond’s value is determined is based on what is typically called the 4 C’s. These four C’s are the diamond’s carat weight, clarity, color, and cut. 
Carat Weight
The weight of a diamond impacts its value. The larger a diamond is, the more it weighs. The more a diamond weighs, the more it is worth.  
The clarity of a diamond is based on the visibility of the inclusions in that diamond. Inclusions are the spots you can see in a diamond. The more you can clearly see the inclusions in a diamond, the less clear that diamond is, and the less that diamond’s value will be. The scale of visibility of inclusions on a diamond are rated from FL (which is considered flawless), to I3 (which is considered in the included range).    
The color of a diamond also impacts its value. This part of a diamond’s value is based on letter categories. The closer the diamond is rated to D the more colorless it is, and the closer a diamond is rated to Z, the more color it has. The less color a diamond has, the more it shines, and therefore the more it is worth.   
The ideal cut of a diamond will reflect as much light as possible out of the top of the crystal, and appear fiery and brilliant. This effect has a direct impact on the value of the diamond. 
Some common shapes of a diamond include round, oval, marquise, heart, emerald, etc