Noel, I believe that this is true primarily for diamonds.
I don’t wish to beat a dead horse, but in my study of faceting, the
process is all about cutting the stone at the proper angles to bounce
the light around inside and back out the top. Light from the back,
if visible, tends to obscure the “sparkle” the faceting creates, just
as light shining in through a window will prevent you from seeing
your reflection off the inside.
The higher the refractivity of the stone, the more brilliance it can
have, if properly cut, so diamonds do this very well and quartz
somewhat less so, but the principle is always the same. Very dark
stones may be cut at the “wrong” angle to allow more light through so
they appear lighter, at the expense of brilliance. “Windowing”,
where some facets at the culet are too shallow, allows light directly
through-- a flaw in the cut. Most of the time in jewelry there isn’t
much light available from the back, except maybe in earrings, so the
window will look dark, not bright, and may show the color of the
metal through, so if this is yellow gold, it may make the stone look
muddy or “off”.
Likewise, gluing a stone in tends to negate the effect of the
pavilion facets, making the stone dull-looking by allowing the light
to leak out the back.
OK, got a little carried away there, but I figure it is better to
offer reasoning and that to just say “Does not!” “Does