I'll add an "amen" to Jeanne's suggestion of Opticon for repairing
a cracked opal. I've used it with great results. I've placed the
stone with the Opticon in a vacum chamber for a few seconds and
then put it under a warm light bulb for over night. Of course the
treatment must be disclosed.
It should be mentioned, however, that opticon has it’s limits. One is
that it can only affect a fracture that reaches the surface. That
seems obvious, but sometimes people think a fracture does this, when
it does not. Also, opticon is applied first as a liquid resin, one
part of an epoxy based formula. The second part is usually applied
afterward. What this means is that the hardener cures the resin near
the surface, where it can reach. But it does not penetrate as deeply
as the original resin applicaion. That application does most of the
work to make the fracture less visible, of course, but those
portions of tht first application that are more than a trace below
the stone surface are not actually cured or hardened. That means
those areas do not act as an adhesive, only as an optical aid in
hiding the fracture. The importance of this is that the OP wished a
treatment to not only heal the fracture visually, but physically as
well, so the cracked part of flake would not come off. Opticon is
only marginally of use in this, because only the surface resin is
cured and therefor acting as a glue. The uncured resin might be a bit
gummy after a while, but doesn’t offer much in real adhesion.
Another product that may be worth trying is a cyanoacrylate based
glue. Sold for gluing glass together, it works well at being
virtually invisible when on materials with refractive indexes close
to glass. The stuff I’ve got in mind (no specific brand, there are
several available) is cured by ultraviolet light, so it only works
with materials that will allow UV to penetrate. Ordinary super glue
probably isn’t such a good choice, because it cures so quickly that
it would be hard to get decent penetration of the fracture. But the
UV curing stuff can be applied in darkened or UV free location
(indoor incandescent subdued lighting), and vacuumed, gently warmed,
or otherwise given time and encouragement to penetrate the fracture.
it’s formulated with a very low viscosity, and the few times I’ve
used it, seems to penetrate well into tiny fissures. When satisfied
with the appearance, one then exposes it to UV radiation (UV lamp, or
just good sunlight). Since this can cure all the way into the
material (assumeing the material is not opaque to UV), then it’s
ability to physically adhere the fracture together is pretty good.
How it compares to opticons ability to fill a fracture visually, I
don’t know, but as a glue, I suspect it might be stronger.
Hope that helps.