I do feel that a completely closed setting results in the stone
reflecting less light back out at the viewer.
When I set a transparent stone in a bezel, I make a “mirror” to put
behind the stone. Since it is near impossible to get a real shine
in the bottom of the bezel can, I cut a piece of sheet metal the
exact shape of the bottom of the stone and put it on a shellac stick
to polish it.
Once I get the mirror shine I put a bit of cork sawdust into the
bezel for a springy cushion then put the mirror on top of the
sawdust and push it down. The stone goes on top of the mirror then
I push and burnish the bezel down on the stone. To my eye, it makes
a real difference.