Glue has a place for stones that you know will degrade through
normal use, and can be expected to need frequent replacing. Pearls,
soft turquoise and opal triplets set in rings for example will need
to be replaced sooner or later. I think it is prudent to design a
setting that makes it easy to replace the stone yet will hold the
stone securely for as long as it takes, and glue is a good option.
I like 5-minute epoxy; it fills gaps with a tough plastic compound,
is predictable, fast setting, and softens with heat at a lower
temperature than superglue. Epoxy cannot be trusted to adhere to
metal or stone permanently - although it does do that very well
sometimes. A rough texture and undercutting the bezel will provide a
key for the epoxy to hang on even if the bond is broken.
Faceted stones can be glued; the important thing is that they can
only be glued in the vicinity of the girdle. Glue on the pavilion
will be visible through the stone.
I use glue in for example, a large emerald full of fractures or a
thin solid opal, hammer set and rattling in the setting. Rattling
because the original setter made sure (with good reason) there was
only the tiniest sliver of metal actually touching the stone.
Resetting or tightening needs great care and is therefore expensive.
I will explain to the customer that the problem can be solved with
little risk or expense, by rubbing epoxy around the girdle and under
the bezel to lock the stone in place; also that the glue may need
replacing in a few years time. If the customer baulks at the idea, I
quote appropriately to tighten or reset, and am quite content if the
customer goes elsewhere!