The most expensive way a diamond can be cut is in the style called "round" or "round brilliant." The cut costs the most because it produces the most brilliance (otherwise known as fire or sparkle) in a diamond....
The most expensive cut of diamond is heart and marquise. These diamonds are the most expensive of the basic diamond shapes because of their tendency to waste a good deal of the crystal's rough form.
Round brilliant is the most expensive diamond cut. A diamond's cut is the one that determines the amount of brilliance and fire that can be seen as it reflects the light. There are many other diamond cut designs but the round one is not common and is more brilliant and hence most people prefer it.
The round brilliant cut is the most expensive diamond cut. The round cut sparkles more brilliantly than any Other cut as it is the most popular engagement ring and accounts for 75 percent of all diamond sales.