It was my understanding that light does not need to come from
behind i.e. the facets on top control the gemstone's sparkle, so a
setting which is closed from behind should still allow the stone to
shine.
For properly cut facetted stones, especially those with higher
refractive indices, this is correct. If the loose stone, with light
shining from the top down onto the table of the stone, looks nice and
brilliant, with light reflecting back to you, then it is capable of
looking the same once set. Stones cut too deep or too shallow, or
with too low an R.I., may still let you see through the stone to
what’s behind it, and with those, whatever is behind the stone will
affect how the overall finished set stone looks. it’s not that the
stone needs light from the back (though for those see through ones,
it can be nice), but simply that those stones function as partly a
window, rather than a reflector by themselves.
However, this fact that well cut stones do not derive their
brilliance from light that comes in from the back, is not the whole
story. An open back may not allow more light to be viewed from the
top, but it DOES let you clean the stone. And a clean stone is
critical. If the back facets have any grease, oils, wax, polishing
compounds, cleaning solutions, or the like on them, or trapped in a
closed cavity behind the stones so they can get on the pavilion
facets, then those dirty facets will not be able to properly reflect
light. The principal reason why drilling through the metal behind a
stone is important is to allow proper cleaning. Even a well done
bezel or flush set stone, despite seeming sealed into it’s setting so
as to exclude dirt from getting behind, will still eventually get
dirty in there. Oils, soaps, liquids, etc, just seem to find a way,
and without an opening behind the stone, you can’t clean it out.
there’s also other factors to consider. Any set stone will appear
somewhat dominated by it’s setting. A bezel or prong or flush set rim
around the stone obscures a bit of the stone, and puts the back in
shadow. The light entering the side or back of a stone may not be
responsible for the brilliant reflections, but a stone opened up on
the back and sides, is simply more visible, more transparent looking,
and will often look slightly brighter or lighter at least, as well
as simply looking slightly larger since less of the stone’s diameter,
girdle, etc, is hidden or covered by metal.
You also say you’re using cheaper quality goods. Be aware that often,
the cutting quality of these is more variable, and as I said above,
if the stone is not fully brilliant (this term refers to the light
reflecting from an overhead source back to the viewer as bright
reflections in the stone) to begin with, the setting will make it
look darker, by putting the windowed area in shadow. If the stone is
fully brilliant and well cut, and the back of the stone is properly
clean, then after setting it should still be just as brilliant.
Peter Rowe