I have cut many an opal, mostly the Coober Pedy variety. My
favorites are from Andamooka, though. (Mine are from over 40 years
ago, and I recall that Andamooka is no longer producing like it did
Your instincts are correct- 1 mm is probably too thin. Cabs than
thin are fragile to set, damage-prone in the finished piece, and in
my opinion just look very flat, especially in the larger sizes. I cut
my cabs about 2 - 3 mm thick whenever I had the material to do so.
Since Coober Pedy opal is mostly white or light grey, I suspect that
the cutter was trying for maximum translucency in the stones rather
than end up with a dead white than tends to smother the play of
I have made a few doublets to salvage thin seams of color in a piece
of rough, and 1 mm or so was the thickness that prompted making a
doublet by cementing on a backing piece of obsidian or black jade to
provide strength and a dark background to bring out the color.
Cutting and polishing was done after the two slabs were joined.