A quick easy way to make your agates look brighter is to make your
bezel just slightly taller than you need and right before you set
it, put a highly polished piece of silver under the stone, face up.
Then set the stone as usual. Make sure the silver sheet fits snug so
it doesn't move around, and that the bezel is well sealed so water
or cleaning compounds can't seep under your stone. This is a nice
trick to add height to a low domed stone too, just add more layers
of sheet. I almost always set my translucent stones with a fine
silver backing and it really reflects light back out nicely and
brightens the stone considerably. An open back is a good option on
big stones; a lot of people like that for metaphysical purposes
because they want skin contact with the stone. Your idea of building
a spacer for a polished piece of silver behind the stone is
interesting, I think it may be difficult to get the polish back up
once it's been soldered, but who knows, can't hurt to try!
Amy O'Connell Jewelry