For me, there are two components to this choice when I approach a
The first, obviously, is aesthetics. Some stones and designs simply
seem to cry out for a "boundary" of metal helping to define them, set
off the color, or integrate the flow of the design. In other cases,
the stone may have flaws on its edges or be a doublet or triplet that
isn't all that aesthetically pleasing except from the top.
The other criteria for me is the stability and type of material
being set. Some stones or cuts are too soft, brittle, or otherwise
fragile to feel comfortable setting in anything other than a
closed-back, heavy walled bezel. Others need a bit of extra
protection on their fragile edges, but can be easily set in an
open-backed, light-walled bezel. Still others are just fine in a
My favorite thing, however, is to create a "blended" setting where
the stone is integrated completely into the piece, so that part of
the piece might act as a prong and other areas of the piece act more
like a bezel. Right now, I'm working on a setting for a gorgeous
doublet opal where part of the opal is "buried" in the setting and
the other part "embraced" by the setting. Very organic.
And then there are the many other types of settings.... gypsy,
channel, pave, etc.
There are no hard-and-fast rules here. The important things are that
your design is integrated and your stone is safe.