I have used Opticon and it has worked for the stones (mostly quartz
based) I use it on, I have not however tried it on Opal. The process
I use is to soak the stone in a covering of acetone to make sure all
oil is gone, then heat my stone (below 200 F and watch for a thin
line to appear around the crack, if it does you still have oil and
will have to wash it again), use a nail to put the resin on it
following the crack, allow the stone to cool drawing the resin into
the crack. Wipe off the excess resin, warm the stone to touch, then
apply the hardener with another nail. Letting this sit for 10 minutes
or until cool and then wipe it off.
The other method on the instructions is to mix the resin and
hardener, apply to the warmed stone and allow it to soak in. I
suspect it will work, but for me cleaning would be harder since when
performing these actions I am also doing something else and I don't
always make it back in a timely manner.
For any residue that sticks on, most the time it will wipe off using
a rag, on soft stones I have used a 1500 grit damp wet and dry by
hand to get it off and then re-polished. Careful on a mat or
semi-gloss finished stone, perform this action before final facing.
I suppose you could use stir sticks, tooth picks or whatever you
find handy, I happened to have nails, they make good dop sticks for
working small stones, stirring epoxy, wedges, spacers and so on.
Bummer on your Opal, let me know how it works. If the cracks are
very very small, wait for another post. Someone may know of a product
which gets thinner. Opticon gets thin when warm but I don't know if
it will get thin enough to get into extremely fine cracks and it
would be a shame to plug the cracks with the wrong stuff. Try and see
which will get the thinnest for you and then go with that one.