Diamonds form crystals in nature, and the crystal shape is called its habit. The most common habit for a diamond is an octahedron, which looks roughly like two pyramids attached at the base. The other habits a diamond crystal can be found in are macle, dodecahedron, and cubic. While these shapes are interesting and pretty, they lack the sparkle that a diamond is associated with. Some material must be taken away from the stone in order to bring out the sparkle of a diamond.
Very few diamonds are "flawless." Most have one or more flaws, such as growth lines, which occur during the formation of the diamond, and pinpoints, which are crystals formed within the diamond. Some types of flaws cause structural instability, such as feathers, which look like actual feathers on the surface of the diamond, and cleavage, which is a major crack. A diamond may need to be cut differently because of these flaws, which means that a diamond may need to be split into pieces to avoid breaking during use or may need to be cut or polished to remove an unsightly flaw.
The most common cut for a diamond is the brilliant cut. There are two main reasons for this. First, it is a regular shape that refracts light very well. Second, it lends itself well to the natural shape of most diamonds, the octahedron. Diamonds that are irregularly shaped may have different cuts, such as the marquise, pear, oval, emerald, triangle and square. The cut is taken into account to try to minimize loss; for instance, a long diamond may get an emerald cut, which can have as little as 40 percent loss.
Because flaws decrease the value of a diamond significantly, based upon what type of flaw it is, a gemcutter may wish to cut to remove a flaw or avoid structural instability, which may remove a large portion of the original gem. In addition, the very nature of gemcutting removes large portions of the gem to create an ideal shape that will refract as much light as possible, which involves removing large amounts of the original material of the gem.