Upload & Sell: On
A well cut stone will sparkle more than a poorly cut one.
That depends ... a well cut stone can be cut to return more light through the table so that it appears more brilliant, or a well cut stone can be cut to return more light through the crown to create more sparkle.
That becomes a design choice for the cutter @ how to style/balance the light return between brilliance and sparkle (fire).
But in both cases, a well cut stone will return the (near) maximum amount it can through one, or both of these two areas ... minimizing the amount of light loss through the other exiting light paths. Light still follows AI=AR @ reflection, refraction and absorption.
Some excellent step cuts can look like a piece of glass with very little sparkle (i.e. emerald cut, etc.). Part of the reason that an emerald cut is considered a mark of a quality stone is that the emerald cut is very easy to see the stones flaws, so those stones with fewer flaws make better candidates for step cuts. Personally, I don't care much for step cuts as they have little character, only to be somewhat pretentious @ "Look at me, I don't have (m)any flaws." But for some folks, they love them.
Diamond cutting is much like lens design in our craft, in that there is always a series of compromises involved in the optical design at how it renders/returns/transmits the light. Our lenses can be optically ideal, full of character or simply poor. Most worthy lenses fall somewhere between optically ideal and some degree of character ... with poor to be avoided. Imo, same goes for gemstones @ how much brilliance vs. fire one will aspire for as a preference ... avoiding the poorly cut or detracting flaws.
While DEF grades (diamonds) are "colorless" and GHI grades are "white", I find the FG (occasional H if cut for maximum fire) to be good targets @ color/brilliance. And for clarity, most folks don't need flawless, but most stones that are SI1 (slightly included) are too small to see in daily wear. SI2 and SI3 (which is really an I) are noticeable and should be avoided. VVS or better are into pixel peeping range and only visible by the most scrutinizing eyes, typically requiring a loupe to detect.
So, for me ... F-G and SI1 or better provide good quality that can be brilliant, with the amount of fire dependent on the cutter's styling.
NOTE: There are many different grading organizations and some dealers/stores make up their own grading criteria (impressive little cards to emulate GIA reports, but aren't). GIA is the most trusted. I've seen stores grade a stone @ F, which compared to a GIA graded stone were more like a J or K ... Caveat Emptor.
As we digress ...