The sparkle is in the stone
Light enters a diamond through the top, strikes the angled facets of the pavilion, and is returned to your eye. How much comes back, and how it breaks up as the stone moves, is decided by the proportions and finish of those facets. That is cut, and cut is finished before any metal is anywhere near it.
So: does the setting affect diamond sparkle? Essentially no. A mediocre stone in a beautiful setting is a mediocre stone. Nobody has ever fixed a dull diamond with claws.
Which matters commercially, because settings are easier to sell than cut grades. A ring photographed under three point lighting on a black background looks spectacular regardless of what is in it.
The three ways a setting genuinely does interfere
Being precise about this is worth more than the slogan, because the exceptions are real and two of them are common.
A closed back. Most settings are open underneath, so light reaches the stone from below as well as above. A setting with solid metal behind the stone closes that off. The effect is modest on a well cut stone, which does most of its work with light entering through the top, but it is not nothing — and it is worth knowing whether a bezel you are considering is open or closed at the back.
Obstruction. Heavy claws and a thick bezel rim cover part of the crown. That does not reduce the sparkle of what remains; it reduces how much of the stone you can see sparkling, which looks the same from a distance and is a different thing. Slender claws show more stone. They also need re-tipping sooner.
Dirt, which is the real one. Tight galleries and enclosed settings trap skin oils and cleaning residue, and a film on the underside of a stone kills its liveliness far more efficiently than any design choice. In practice, more rings look dull because of this than because of anything structural. The entry on daily wear covers it properly.
Setting height is often listed here too, and it deserves less weight than it gets. A raised stone sits further from the shadow of the hand and catches light from a few more angles. It also catches doorframes. The optical gain is small and the practical cost is real.
What a setting actually changes: how big it looks
This is where the design work genuinely pays, and it is a matter of proportion rather than optics.
A halo surrounds the centre stone with small ones, and the eye reads the whole cluster as the stone. It is the most effective way of making a modest centre look substantial, and the effect is entirely real from any normal distance.
A bezel can go either way. A rim of metal that sits flush and close can make a stone look larger by extending its outline; a heavy rim overlapping the crown makes it look smaller. The same setting type, two different outcomes, decided by how it is made.
Band width does more than people expect. A narrow band gives the stone less to be compared against, so it reads larger. A wide band does the reverse while making the ring itself more present.
Claw count and style. Four claws leave more of the outline visible than six and make a round stone read slightly squarer; six hold more securely. Thin, well-finished claws disappear at arm's length.
Elevation, again. A stone standing clear of the finger reads larger than one sunk into it, for the same weight. This is the one case where the raised setting earns something, and it is an appearance gain rather than a sparkle one.
None of this changes the stone. It changes the impression, which is what anyone looking at your hand is actually receiving — and the forthcoming entry on shape and finger proportion takes the same argument further, since shape affects apparent size more than setting does.
The trap this creates
If a setting can add apparent size but cannot add sparkle, then the money has an obvious priority and most people get it backwards.
Buy the cut. It is the only variable that determines how the stone behaves in light, it cannot be added later, and it is the thing that will still be doing its job in forty years. Then use the setting to solve for size, for how the ring sits on the hand, and for whether it survives what you do all day.
The common error is the reverse: paying for a high colour or clarity grade, then choosing a setting that hides a good deal of the stone, and concluding the ring is somehow less impressive than the certificate promised. The entry on the four Cs makes the same point from the stone's side.
What this means when something is being made rather than bought
The advantage of a commission is that these are separate decisions taken in the right order. Specification first, against what you actually care about. Setting second, against the hand it is going on and what that hand does all day.
If you want the stone to look as large as possible, say so plainly — it is a design brief, not a confession, and there are several honest ways to deliver it. If you want the stone protected because you never take rings off, that points somewhere else entirely.
Tell us which of those matters more, and the setting follows from the answer rather than the other way round.