These are known as "commercially" cut diamonds. Diamonds that are not cut to perfect proportions, i.e. commercially cut, will be either too deep or too shallow - this is undesirable as light will be lost through the stone and it will look more like a piece of glass than an expensive gemstone.

Ideal and premium cut diamonds can be more expensive than commercially cut diamonds but it is also worth noting that a good cut can camouflage a lower clarity and colour grade, making the diamond look bright and light regardless of these factors. Compromise is nearly always necessary and it's better to lose a factor that won't be noticed as much.

Do not confuse cut with shape. There are many shapes of diamond and each will require skilled cutting to maximise its brilliance.