I'm going to fill the bezel tightly with opal chunks then fill it in
with black Durenamel
Doesn’t sound like a good plan. To properly inlay opal, either as a
single piece, a buildup of several pieces, or as a mosaic, the
material is sliced to the approximate thickness and then cut to fit
the mounting or pattern. If it is being set in a deep bezel, you
will make a doublet then cut it to fit the mounting.
If you just try to glue chunks in, you will get an uneven thickness,
and the points will contact the metal and you will not get the black
backing you want. Also when you grind down the opal to the desired
height you will end up with an unpredictable pattern.