If you look and compare round brilliant cut diamonds and other fancy shape diamonds, you will realize round brilliant cuts are much more sparkly. Round brilliant cuts are typically 20-30% more expensive than other fancy cuts.
Rounds are cut mathematically proportional. To cut to its perfection, there will be more wastage in the rough.
I highly advise you to only look for EXCELLENT cut round diamonds for maximum brilliance. This is where the light goes into the diamond and bounces back up to your eyes to show scintillations.
If your round diamond is cut too deep or too shallow, the light is lost while traveling through the body. If the diamond is cut too deep, it will look dark and lifeless. If it’s too shallow, in another term “Spread Diamonds”, it could look bigger than it’s carat weight, but the sparkles are lost.
Other fancy shapes are not cut in a mathematical way like the round brilliant cut diamonds, hence there are no cut grades for fancy cuts on the certificate. It is due to whether what kind of shape you like because each cut could come in different proportions. For example, Oval Cut: you could get a fat and short oval shape diamond or a slim and long oval diamond.