A diamond cut is a style or design guide used when shaping a diamond for polishing such as the brilliant cut.
The cut has several different names like, The Ideal Cut and Hearts & Arrows. Don't be fooled, they are all the same. A Diamond has 57 or 58 facets (sides) depending on if it has a culet or not (whoch is the bottom point of the diamond).
The cut of a diamond has three main attributes:
1. Brilliance: the light reflection from a diamond
2. Fire:the way the light disperses in a diamond
3. Scintillation:the sparkle of the diamond
Before computers and fancy cutting techniques were used, it was common to have the culet be a facet. This style of cut is called an Old Cut or Euro Cut. Depending on what style of ring you are looking for, Old Cut diamonds are usually found in antique pieces.
The most common cut (and world’s most popular) is round, which also happens to be the most expensive shape. There are Fancy Cuts, which are any shape that is not round. Some examples include Emerald, Princess, Oval, Pear or Heart. These cuts are not graded based on their cut because in reality, there is no ideal or correct cut for these shapes. There is however, better proportions and symmetry that will make the diamond sparkle more - ultimately it comes down to your preference.
For example, when looking at emerald cut diamonds, some people like a more square shape when others like more of a rectangle. What I recommend, especially for fancy shapes, is going with a higher clarity. This is because of the way fancy shapes are cut; sometimes the inclusion can be eye visible (meaning can be seen with the naked eye.
If we are talking about purchasing a stone with the most sparkle, your best bet is to purchase a Round cut stone. The angles and proportions of each facet help improve the brilliance/fire of the stone. This is done by keeping the light inside the stone as long as possible. Stones that have poor proportions/cut grade have light leakage and appear dead. This can be seen in lots of inexpensive, lower clarity stones. If you want the most sparkle, choose a stone that has an Excellent/Very Good cut grade. This will insure that the stone retains as much light as possible based on the size and shape of stone.